Hello Colonel - I would like to know that as well, which sometimes would save creating a swap with someone, that beforehand you could know that it would be turned down.
However, before you create a swap you could look at the Profile of Mr X and in there is a section, which they can fill in called
What he/she collects. So if Mr X has written for example that they collect Euros and you do not have any, then even though Mr X has some coins you want, you would know that you do not any for them.
Another way is in the Topic below, where in Advanced Search there is a part called
Your Collection: in/out/both - but I am still not sure how that works - either for Mr X or for someone looking at his Profile. Hope it helps
https://en.numista.com/forum/topic5216.html
After writing the above, there are other Topics, involving a wish/want list - which for yourself would be handy as if Mr X had created one you could look at it, before starting a swap. This facility has
not been added to the site, and is just a discussion. Would be handy if it
was added.
https://en.numista.com/forum/topic3536.html
https://en.numista.com/forum/topic2055.html