i need help with the transfer case on 1999 GMC Sierra 2500

how can you check to see if the transfer case for my 4x4 needs fluid

there is a plug that you unscew on the transfer you stike your pinky finger in the hole and you shoud fell the oil if not then you shoud add. the plug should be about the center of the transfer
thank you I got it done still hear a noise like a whistling noise i don't know if it is totally fixed i drove from ms. to nc. pulling a u haul tailor when i came down off the mountain it starting acting like my emergency brake was on.. a man inside of mc donalds helped me and said he thinks this was what's wrong.. Gm told me to bring it in to be serviced but i am up in the mountains of Nc. not sure if i can make it back or not crossing my fingers that this whistling noise will go away.. thank you denise