There are two ways.
1. Replace it and see if the problem goes away.
2. Uses SDS (Star Diagnostic System) to check sensor.

__________________
John Shellenberg
1998 C230 "Black Betty" 240K

http://img31.exs.cx/img31/4050/tophat6.gif

UPDATE:
I took the car to a dealer. They ran star diagnostics and pulled up a huge list of error codes, and the tech also said something about reading low voltage. He asked if I tried jump-starting the car at all, to which I replied "no." I then re-explained the symptoms and he said they would try to reproduce the problem. A couple of hours later, he called back and said they were able to reproduce the starting problem (continuous cranking) and he said that "based on what [they] know it's probably the crank sensor." They weren't able to find a code for that on diagnostics and he wasn't very convincing. But given what almost everyone has said here and what I've read elsewhere, I told him to just go ahead and replace the crank sensor. They're going to replace it tomorrow morning and test it out. Could a faulty crank sensor cause the low voltage readings? Also, he quoted me $375 for parts/labor including the diagnostics, is that about right? Thanks again.

I had the same problem last week. Thanks to this post, it is solved.
Paid $100 for the part and $97.50 for the labor.