Ball Python Heating and Humidity

Question
Hi! I got a young ball python that is about a foot long two weeks ago and was wondering about its heat and humidity. I keep him in a twenty gallon tank and have read that their cages should be 50% or so humidity, but I do not know how to monitor that. Is there anything that I could buy to test the humidity? And what is a good way to keep the humidity from escaping from a sliding screen lid? Thanks!

Answer
There are monitors you can buy, you're local pet store should have them.  If not, or maybe better yet, try www.lllreptile.com  Aluminum foil or plastic wrap over a portion of the lid can help keep in moisture, but usually just having a good size water dish will do the trick.