How to check if auto 4WD is working

This has the automatic 4wd. I guess that's what you call it. There is no selector switch. But it has a 4wd indicator light (idiot light) in instrument panel and has a 4wd imprint design in dash. No selector switch or anything though. A few months ago the 4wd flashed indicating the tone ring was a problem and I found the drivers side ring broke. I replaced it and the ABS sensor. I was told at one point the 4wd light had come on when it was being driven but can't remember if she told me what road conditions were plus I can't remember if this was before or after I replaced tone ring. We live in Northeast Pa and have had plenty of snowy and icy roads. We both have driven it and have never seen the 4wd light come on at all. And conditions definitely sucked. The wife has stated the vehicle has slid a few times on some hills. How can I find out if it is or isn't working. I have a Hayes manual and looking in the chassis electrical schematics I noticed a wire leading to what is listed as. 4wd control module. There is no other reference to this module anywhere in the book. In fact not much if any info at all about this type of 4wd system. Can you help explain this system and how to troubleshoot it?