Light bands are 50lbs when stretched to 6ft. (Per the Jumpstretch web site.)
Take a look at the top of your lift and guess-timate how far the bands are stretched. You can get a good ball-park figure.
EDIT TO ADD: I believe B-Fold uses the DB method to check his. He attaches his bands to DB's to hold them down, and when the DB juuuuust holds it down, you know pretty much how much tension you have. Same with plates.