go to www.millertool.com AND BUY ONE-then ask funkychicken over on LX FORUMS FOR the correct way to check it. if i remeber right you have to have the trans hot to 180 degrees and the stick should read at the 75% mark also the snap on truck's now have them.
there is really no need to check tranny fluid, unless you notice a leak, and in that case you r taking it to the dealer. And if you change the fluid your self, i guess just put in the correct amount it states in the service manual. I changed my own and than went to the dealer just to dip it to see i had a good level. i was at the dealer for 3 min. hope this helps