hook then up to a reciever and battery and try to move them, the problem servos will show themselves out pretty quick,also a simple rattle test check the servo horn to see if there is any slack or movement.

you can also open them up,just pull the screws pop the cases open, and you can check for dammaged gears or exessive dirt in the servo,for example a servo that was used for RC cars will have alot more dust and dirt in them if theres exessive dust in them and the gears look good, the computer cleaner, "air in a can" can help get any debris out of them,if you notice any problems its not worth the 15$ servo to use them in a plane.

w8ye nailed it the 3004 is a prety good servo,I use alot of them the 3003 is a bushed servo, instead of bearings, there are bushings that work well enough, but can wear out and get a little loose,I have a few of these as well but use them mainly for throttle.