If someone is tracking how BTC are being spent or transferred between wallets, is it possible to concretely determine which transfers constitute conversion into fiat currency at an ATM, and possibly a specific ATM? If so how?
That would depend on the owner/operator of the ATM, and your relationship with them. Normally, you will need to register with a Bitcoin ATM company in order to use their machines. This is so that the company can comply with local laws and regulations, just like exchanges need to comply. Effectively, Bitcoin ATMs are not "automatic teller machines", but rather "automatic exchange machines".
By looking at the blockchain alone, it would be hard to determine whether a transaction was received by or sent from an ATM. However, if the party operating the ATM were to open their records to whomever is tracking, then it would be trivial for them to report who used the ATM, where it was located, and the exchange rate you received at the time.
Basically, it's the same as using an exchange.