How to tell if an unrecognized, unclaimed address you sent BTC to is actually a "made for CHANGE" address? And if you discover a CHANGE address with unspent $$ in it, how can you retrieve the funds?
When you send a bitcoin transaction, an entire UTXO is consumed, and any value in excess of what you paid out will be sent to a 'change address' that your wallet controls.
That is all a 'change address' is, it otherwise is no different than any other address. Reasonably, only the participants of the transaction in question will know that an address is a change address. Your wallet will automatically create and utilize change addresses, there is nothing extra you need to do in order to use or claim the coins.
And if you discover a CHANGE address with unspent $$ in it, how can you retrieve the funds.
If you have the private key for that address (ie, it is part of your wallet), you can spend the funds. If it is just a random public address that you have discovered or been shown, then you will not be able to spend those funds. Simple as that.
Note: if someone is requesting some sort of payment or fee to 'spend/recover/etc the funds', you can simply ignore them. That is a form of a common scam.