I'm assuming you have a concrete floor since you're asking about laying poly down ...
Cut a 2' x 2' piece of poly and duct-tape ALL 4 sides COMPLETELY down to the floor. Wait 2 days. If you simply have some condensation, then you have some moisture. IF you have large dropletts, then you may have a bigger problem.
You may need to 'level' your floor with a floor leveler. You'll then want to install 'sleeper' joists on the floor. Then, on top of that, a subfloor (of sorts.) If you only have condensation, then DON'T cover the floor w/poly. Let the flooring below breath and use a dehumidifier to remove any excess moisture.
Personally, if the basement is below grade, I don't recommend this type of flooring. In short, consider berber carpeting or indoor-outdoor carpeting. I like to leave the floor with the ability to breath.
My best to ya and hope this helps.
Jay J -Moderator
PS: For carpeting, you don't need to level the floor or install sleepers.