Is there a way to search a certification number on PSACard.com to see if a specific card is part of someone's set?
Once at the PSA website, click "RESOURCES", upper right.
Then scroll way down and on the left side you'll see "cert verification" Enter your cert # in the box.
You can verify the cert is valid, yes, but there's no way to see if it's part of someone's set or who might own it.
The only way is to attempt to add it to your own inventory. If it's not part of someone else's set and you don't own the card you could then remove it.