1) Either you look at what you are doing on your computer, and how secure it needs to be, and conclude that nothing in your life is totally secure (post can get intercepted, telephones tapped) and that actually, what you are doing is not so high risk that the current level of certainty is unacceptable. So there is no problem.
2) Or you enlist the help of experts in malware forensics, to see if they can determine if you were ever infected, to a higher degree of certainty than here - but still not absolute.