If the speaker has a make and model number you should be able to find the nominal impedance and maybe the minimum impedance in the specs or in a review.
Your receiver has to be able to support at least the nominal impedance. If it supports the minimum impedance it will drive the speakers and sound better than if it doesn't. Worse case it will go into protection, blow a fuse, or blow up if it isn't happy with the load the speakers present.