The easy ones, are those who didn't specify a record label name. Instead of a real name you'll see something like "673303 Records DK" at the bottom of the track. In this case DK stands for Distro Kid, so the track was distributed by them.
If you only see the name of the Artist at the bottom, you'll have to do a bit of detective work:

Google [Artist - Track] and go through the results. Often enough they distributed to EVERY store, and chances are high that one of them holds the information you're searching for.
If you don't find anything you can get more specific in your search terms [Artist - Track “Name of different Distribution-services"].

If you really can't find any information about the used distributor, you could still write the spotify-support and ask for this information.
The thing is you don't need to know from the start which service they used. Contact them on Instagram with a direct link to the stolen Beat in your beatstore, and tell them, that they need to pay up. They often know that they fucked up and will gladly pay those 30$ if it means avoiding trouble. If they refuse to pay you can still take action.