You won't scramble a PS1 engine with a dead/weak battery if you don't leave reset mode. Fire the engine up and let it charge for a couple of minutes then kill the track voltage and make sure the shutdown sound sequence completes in its entirety. Get you ear right up to the speaker and listen for three taps after the sounds stop. If you hear that, the battery is has some capacity to it yet and should hold enough charge for your test. Now restart the engine and let charge for a couple more minutes and then begin the moving test. Also, make sure the smoke (if equipped) is switched off before you start any of this.
If you PS1 engine has been sitting for long while, I've seen the batteries get so dead that they reverse polarity which can cause magic smoke so while you are charging it, also keep an eye on it.
As for PS2, just go to town. If the battery is dead and you try to move, it will just shutdown and turn back on without moving. MTH learned from the PS1 design flaw and made sure dead/weak batteries won't cause major problems.
Also with PS2, if the test track does have DCS, you can ask for an odometer and hour meter reading to give some idea how much runtime the engine has had.
Last thing, I'd avoid buying the early 5 volt PS2 engines, they suffer premature failure due to defective capacitors used in the early 2000's. They can be identified by the Round charging port (or no charging port) on the bottom of the locomotive. I stick to later 3 volt PS2 engines with a two pin square charging port.
