Once you've taken the lenses out of their blister pack or vials, there is no way to tell which is which. That is why you should never take both of your caps off of your case at the same time. Sometimes if your eyes are different, but close in prescription, it can be really tricky to tell if you have switched them. When in doubt, you'll have to throw them out and start over with a new pair.
Patients who wear RGP lenses have the option to have a dot placed on the right one to keep them straight. Can't do that with soft lenses though, unfortunately. Sorry!
Me - Board Certified Optician