How can I definitively check that my 4wd is in fact not working

Started snowing here. Put my 4runner into 4wd and going up hill it still has its rear sliding out like crazy like I'm still in 2wd. It doesn't feel right, but I cant tell if its my imagination or the 4wd is actually out. Is there an easy way to check definitively that I have lost 4wd?

I tried gunning it in 4wd in a snowy parking lot to see if the front tires would through up snow but they didn't seem to be spinning all that much. There was also no snow thrown up under the wheel well, but that was the same for the rear tires that do spin.

Also my 4wd light is out, it might have something to do with the loose wire that came undone. The wire that is loose doesn't look original though, where under the dash does the wire controlling the 4wd light connect with the instrument cluster?

 

Put it up on jackstands and put it in 4 wheel drive. Any 4wheel drive vehicle I've owned ( which has been a lot) it's pretty evident when it's in 4wheel drive. There's a procedure for switching the vacuum lines on the ADD to verify the actuator is working. I've never done it but you could prolly Google it.