How can i check if a particular sound I used in my video was copyrighted?
In nearly all countries, copyright protects creative works automatically. Some exceptions and restrictions apply, but unless the author has voluntarily added his work to the public domain or the copyright has expired (common terms are seventy and ninety years after publication or after death of the author), it's safe to assume all songs are copyrighted, including the royalty-free music you've been using.

Whether or not it's okay to upload material that contains this music to youtube, that would depend on the author's consent. Sometimes, the music is released under a blanket licence that allows redistribution. If not, contact the author to obtain permission.