Basic Primer on examining a bat
Spotting a bat that's had a visit to the doctor can be tough. A lot of these guys are GOOD, and can certainly cover their tracks very well. However, not all of them are perfect, and they sometimes leave telltale signs of their work.
Here are the major categories of doctoring, and what to look for:
ABI - Rolling/Vising/Hammering:
ABI stands for "Accelerated Break-In." It is a category of actions that doctors take upon a bat to abnormally accelerate the break-in period of a composite bat. The three most common methods of ABI are rolling, vising, and hammering.
Rolling can be very difficult to spot, but there can be some telltale signs. The newest Easton bats such as the Reveal and the Clarity will delaminate in such a way that it will be obvious to the umpire that the bat has been rolled and not simply "broken in." Rather than delaminate in dime-sized spots through normal use of the bat, you'll see it delaminate in long streaks. Other bats do not have this delamination technology, so spotting rolled bats will be tougher. Occasionally, you can feel the ridges between the areas that were rolled, but it's rare.
Vising can also be done to accelerate the break-in, but it is significantly less common. Basically, the bat is put into a vise where it is squeezed to compress and thin the composite fibers of the bat. A vise will often leave identifying marks on a bat, which is one more reason why it's done less often.
Sometimes, bat doctors will overdo it when they roll or vise a bat. They'll apply too much pressure in the process, and the bat will occasionally crack. It all depends on how aggressively they roll or vise the bat.
Hammering is the process by which the bat is struck with another object, typically either a rubber mallet or a "ball mallet." Basically, a ball mallet is a handle with a softball attached to it. Sometimes, you'll even see guys hit their bat against solid objects such as trees or poles. It's the same thing.
Painting/Graphics:
This is the process by which one bat that is not legal is made to look like another that is legal within the association.
When a bat is painted, it is often sanded down just a little bit to thin the bat enough for the paint job. The bat is then primed and (very artfully, I might add) painted to look like another bat.
Spotting a painted bat is a little easier than spotting a rolled bat. Look for paint on the end cap. There should be none whatsoever, as the end cap is inserted AFTER the paint's dry on a legitimate bat. Use the ASA approved bat ring, as painting a bat will increase its thickness. Check for drips in the paint or clearcoat.
Lately, bat doctors have been using very thin decals to accomplish the same goal. Search on eBay for Freak 98, and you'll see a few of the decals being sold there. These decals cover up the original graphics of a bat and make it look like another bat. The doctors will then put a thin layer of clearcoat to finish the effect. Check for raising in the graphics that might indicate that it's a decal, and check for drips in the clearcoat.
Any bat can be painted to look like another bat. Most commonly, you'll find bat doctors taking a Miken Ultra II and making it look like a Miken Freak 98, and I check these bats a little more closely. Check the graphics to make sure they're on straight. Check for relative consistency in the wear - if the bat looks like it's been through a war, but the graphics look brand new, there might be something funny going on.
Endloading/Knobloading:
Adding an endload (weight at the end of the bat) can increase the amount of "pop" a bat has, often making the bat exceed the 98 MPH limit ASA sets as their max performance mark. There are two ways of doing this: going through the end cap and going through the knob.
Check around the end cap for evidence of tampering. If it looks like a screwdriver or chisel has been taken to the end cap, then it may have been popped off. However, be careful with this, as small rocks can also get wedged between the end cap and the barrel, causing a similar effect. Check the end cap to make sure everything is lining up correctly. Easton's newest bats will have "EASTON" stamped over both the barrel and end cap, so any misalignment should indicate that the bat should be removed. Also, some doctors will put the bat in boiling water to loosen up the adhesive, making it easier to pop off the end cap. Check for discoloration or bubbling in the paint that may indicate it's been boiled. Lastly, check for extra adhesive coming out from the end cap.
Lastly, examine the knob for evidence of tampering. Some safety knobs will ship from the factory with a small hole in it. These holes are perfectly centered and perfectly round. A hole in the safety knob from a bat doctor will not be so perfect - it may be slightly off-center, and there may be burrs around the edges.
A forewarning: taping the safety knob, BY RULE, is legal in ASA.
Knobloading isn't as common, but may still be done on a bat to increase the amount of bat speed. It's still doctoring.
Shaving:
The most aggressive form of bat doctoring is shaving. Shaving thins the walls of a bat to give it more of a trampoline effect when hitting the ball. It also significantly shortens the lifespan of a bat.
There are two major ways a bat may be shaved: on a lathe or with a drill. Most bat doctors are pretty lazy in this regard and will use a power drill with a wire brush to scrape out the walls. The more "professional" bat doctors will use a lathe to scrape out the insides more evenly.
Check the end cap in the same fashion I mentioned above, as this is the only entry point through which a bat doctor may shave the bat.
Some players insist that sound be used as a determining factor in judging whether a bat's been shaved or not. I don't buy it. Sure, shaving a bat will cause a change in the sound it makes when striking the exact same ball at the exact same temperature and humidity with the exact same amount of force and in the exact same location on the barrel, but... Well, you get my point. Do not use sound alone as a determining factor. Check for tool marks on the safety knob.
Shaved bats tend to fail after only a short amount of time, and when they do, they fail in a VERY catastrophic fashion. In other words, they blow up all over the field. Pick up the pieces and examine them. If it's absolutely clear that the insides have been scraped, it's probably a shaved bat.
Sometimes, we can't find evidence of bat doctoring until after the bat has already blown up on the field. I don't know if you call any ASA, but ASA requires umpires to confiscate all pieces of a bat upon a catastrophic failure of the bat. If a player refuses to allow this, eject them. Collect all pieces of the bat and turn them in to the UIC for closer examination. They may end up sending the pieces off to the manufacturer if the initial examination is inconclusive.