Get it in a dry pavement parking lot, put it in 4wd and try to do figure 8's. You'll feel binding if its' working. And if it is, don't do this very much, it's not particularly good for the t-case but won't hurt it for a little testing.
put in 4 low and drive it you'll hear some new sounds, and it will be much slower than 4 high or 2 high. when you turn the front end will "skip" you'll know it when it hapens. listen for any thing that sounds bad. 4 low is real loud but you want to listen for pops and bangs. even more if it has auto hubs they suck. you'll know. if they are bad when you turn you'll hear them pop or bang. guess it all in how you would take the sound. crawl under it and look for leaks or dammage to related parts. good luck find'n your new toy!!!!!!