Checking transmission mount

I searched through 20 pages but didn't find an answer. How much play should be in a transmission mount before it's "no good"?

While the rear DS was pulled and I was replacing u-joints I had a look and was able to grab and shake the transfer case as much as 1/2" in any direction. To the point that the transfer case linkage was clunking around.

Seems like the mount is shot but for curiousity sake I'd like to know the proper way to test this.

How many miles ? They are cheap enough that I consider them routine 150,00+ mile maintenance, and replace them when the motor mounts are replaced, or when doing any service work that drops the crossmember.

Only way to know for sure would be to pull the crossmember. Support the trans/t-case with a jack stand, jack, or other. Remove the trans mount bolts from the cross member, and remove the crossmember bolts. Drop cross member. More than likely your mount is falling apart. A "simple" visual inspection of the mount, without removing at least part of it, is not good enough.
Although, I guess you could always pull the trans mount bolts from the crossmember and jack up the trans without pulling the crossmember.
But, they are cheap enough. Go to your local parts store and buy a replacement. Or, if you can afford it, pick up one of those new Stinky Fab mounts.