Ways to estimate the rated current of an induction motor without nameplate
It's useful to measure the winding resistance. 

If you use other methods, this method can be used to cross-check the results you get from other methods.

The voltage across a winding divided by that resistance gives you the stall/starting current in that winding. Adjust for star/delta configuration as appropriate. 

I(stall) = V / R.

If you have only 2 windings of different resistance, you have a single phase motor : take the lower resistance winding as the "run" winding and use that figure; the other winding is just the start winding.

Now the stall current will typically be between 5x to 10x the rated current, corresponding very roughly to motor efficiencies of 80% (usually small motors) to 90% (probably over 1 horsepower). This is only a guide : if your motor is pulling a train, it may be designed for slightly higher efficiency.

I(rated) = I(stall)/10 up to I(stall)/5.

The same motor may be rated for both 80% efficiency (stall current/5) for (say) 20% duty cycle or 15 minutes, and 90% efficiency (stall current/10) for continuous duty. These are approximate electrical efficiencies; magnetization losses, bearings, friction, windage etc will subtract from them. To estimate those, measure current when running unloaded.

So if you're reverse engineering, with no better source of information, start in this range (say at stall current/10) and use additional parameters such as operating temperature to refine these estimates.

Note that to start the motor successfully, wiring and breakers must be rated to supply the full start current which may be in the 100A region : this does not imply a 100A breaker, but a "motor rated" (class C or D) breaker for your choice of rated current, which will momentarily supply starting current without tripping.