Get motor ratings from unknown DC motors
A multimeter won't tell you this and realistically nothing will other than either the maker's details (not available) or trial and error. Trial and error means testing each one and finding out how much torque and speed it can generate and how hot it might get. You need to make an estimate of how hot you dare run the motor and hope for the best.

I hope someone can help you more than I can. A typical smallish DC motor rated for 12V will have a measured dc resistance of possibly 1 or 2 ohms when not turning. At 24V this might be between 5 ohms and 10 ohms so no real coherent measurement on resistance will work.