Check oil level

What is the proper way to check transmission oil level on a 99 suburbsn?

You should have just included this with your other post:

I will also now suspect that you are having transmission problems. If you don’t give people the full story you are likely to get bad advice from lack of info on the part of the “advisers.”

The correct procedure is in your owner’s manual. The norm is that engine should be at full operating temperature, left running at idle, with the trans in park.