How can I tell the difference between a pair of clear contact lens?

Try wearing them. Unless there substantial difference between the curvature and power of the two lenses trying on the lenses and wearing them either way may help you decide which one is more comfortable on which eye. It is doubtful that wearing the rght lens on the left eye or vice versa will result in any injury and the user can generally tell which configuration is more comfortable and provides optimal vision.
Tinting. Sometimes, lenses are differentially tinted to correspond to each eye. Say a lightly tinted blue lens for the right eye and a green for the left. The tint is so light such that it is not noticed.