It's impossible to deny BTC payments, you have probably provided your btc adress somewhere. It's extremely improbable that they typed in your adress randomly.

Only reason why you should worry is if published your private information and an that btc adress. But in this case it's just $50 and you can keep it unless you would like to return it to the sender.

There is usually no easy way to see where it came from, it's kind of the point, but you can often rightclick in your wallet and get see transaction information, sometimes you can trace it back to a known adress and this might be a clue.

You could also google the adress that recieced the money, se if it's somewhere online.

If you don't want unspeccified money you can send them out into the void, but you cannot refuse transactions.