As long as you print them out from an acceptable website, you'll be fine. (For example, coupons.com, smartsource.com, redplum.com, couponnetwork.com, etc)
As far as if they will be accepted for the product you are buying, you can match the UPCs - the first group of 5 numbers on the coupon should match the the first group of 5 numbers on the product. As long as they do, the register is not suppose to beep/reject it.