I have a motor which has no V, I marking. I think it is either 12VDC or 24 VDC. It is @ 2" dia, 3" long and has the markings:
MY6812B,
B07053.
Trying to see if it is 12 or 24 VDC but not sure how to determine.

 

Post a photo.

It depends on the size of the motor of course, but the windings are usually designed to attain a safe no-load RPM with a certain applied terminal voltage.

The motor will self-destruct when run at too high RPM.

Try a variable DC supply, increase the voltage slowly, stop when you reach 6000 RPM, which should be safe for a small DC brushed motor.

 

Google shows a 24vdc 100w motor.
Manuf. Unite Motor Co.
Max.

 

Google shows a 24vdc 100w motor.
Manuf. Unite Motor Co.
Max.

But it also shows it as a 12VDC https://www.google.com/search?q=MY6812B&ie=utf-8&oe=utf-8&client=firefox-b-1

 

Looks like it has two voltage versions?
24 Volt 100 Watt MY6812B Electric Motor with 9 Tooth #25 Chain ...

One site mentions the 24vdc runs at ~2700rpm off load, which sounds about right for that motor.
So a RPM check on 12v would probably confirm the voltage rating.
Max.

 

But many of those cheap small motors can operate at many voltages. They aren't made to last.