I dare say if you leave the DC there long enough you'll get smoke! If we're talking 100A, then we're talking a sizable motor methinks.
Sun,
As Erik has suggested previously - use a 'DC current transformer' - actually I'd call it a hall effect current transducer. Do a Goggle on this and I think you'll find something suitable from LEM and Honeywell. These give you a sizable output so you don't have to delve into microvolts.

If you're going to use a semiconductor to switch that sort of current then you'll have to look very carefully into snubbing the inductive kickback. I don't think zeners come large enough to control what you're talking about - look at large varistors and/or big R & C. Do the math on how much energy is stored and that will determine what method(s) you have available.

If the object of the exercise is to measure the resistance, there are a number of methods for doing this other than passing large currents through it and measuring the voltage drop - this is brute force at its best! I have a little ESR & low ohms test box that measures 1/100ths of an ohm by passing low current pulses - I don't understand how it works but it does! Check out www.Jaycar.com and search for the esr & low ohms meter kit. It should have the magazine reference so you can look at the article which explains the technique. No high currents involved - it runs of a 9V pp3 battery. I use it for testing for shorts on pcbs and measuring switchmode psu capacitors. It is deceptively simple and effective.

All I can suggest is to do your math and read the device specs carefully - you're talking about a lot of current and energy and that means smoke and a lot of dud semiconductors if you're not careful.