Can we determine which one of the referenced outputs corresponds to a cashback transaction in BTC transaction scripts?
First, the common lingo for these "cashback transactions" is "change outputs", and "chain analysis" is a thing whole businesses have sprung up around. There are a few indicators that allow making educated guesses on which outputs may be change, e.g. 


round numbers (e.g. three outputs with round amounts and one not)
wallet fingerprinting (e.g. all inputs are multisig, only one output is multisig)
address reuse (some wallets send change back to an address previously used by the wallet)
unnecessary input heuristic
probabilistic graph search
…


The Bitcoin wiki has a whole section in its article on privacy describing approaches for change address detection.