testing unknown motors ac/dc

You should be able to tell from the # of motor connections.  DC should only have two and the AC will probably have three.  If you have a variable PW transformer like TW, ZW, etc you could put a diode in line with one of the transformer outputs and dial up the voltage slow.

DC Purrs like a kitten  and the AC will smell great (Ozone) when they run.

A DC motor will only run on DC and will be damaged by AC.

What you are calling an AC motor is more properly called a series wound universal motor. That will run on AC or DC with no harm.

Do not run them on too high a voltage.

Check for a permanent magnet in the motor. Hold a small screwdriver next to it to check. If so then it's DC.

Yes, what we refer to as "AC" motors are actually "Universal Series Wound" motors which have a wound field and will safely run on either AC or DC. OTOH, if AC voltage is supplied to a permanent magnet DC motor it will not turn but rather will hum and try to reverse 60 times a second. Older Permag motors can have their magnets very quickly damaged by such. So you can safely run it on DC but that won't tell you which type of motor is in there.

If the models you are buying have "AC" (Universal) motors they will also have some kind of "reverser"  to reverse the direction of current in the field winding (with respect to the current in the armature) and you should detect this while running such a motor on DC (either the familiar two-position or three-position cycling).

Eyeballs on the motor will tell very easily. If it is an open-frame motor (you can see the armature and the field pieces) and you see a wound field it is a Universal (AC) motor whereas if you see some kind of permanent magnet it is a straight DC motor. If it is NOT open frame ("Can" motor) and has TWO electrical connections it is DC whereas if it has FOUR connections it is "AC".