The "online opinions" of others who don't know your wife are not likely to be any better than your own.
If you want an unambiguous answer, buy a sperm-testing kit online (they're fairly inexpensive) and find out for yourself. You can then make an informed decision re. what to do if it turns out your wife's panties are filled with another man's semen.
For instance, once secure in your knowledge, you could masturbate vigorously while imagining what your wife and her lover might be doing together.
As they say: "The truth will set you free." Go for it...