Been using royalty free music for some time, however youtube shows that I am using some copyrighted material. So just wanted to know if there is any way by which I can check if a song is copyrighted or not?
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.
"Royalty free" is funny that way, because the content is actually copyrighted - but you're given permission to use it.
If you have your source, and can point to a licence that states 'royalty free' you should be able to get Youtube to reactivate your video. You'd probably need to give credits where credit's due in your about text.
The royalty free music is still copy-written. You have purchased a right to use it without paying royalties, but YouTube doesn't know that you have those rights. You need to provide the necessary documentation to YouTube that you have the right to use it there (license statement from your purchase) and they should re-activate it for you.
They do this to make sure that someone else doesn't grab the music from your video and use it in their own without paying for it. (Since generally royalty free video is licensed to the individual and can't be reused by other people you give it to.)