How can I tell when an abbreviation is an acronym?
The simple answer is, you can't. If you are having a conversation with someone who uses an acronym, the only way you'd know is if they spelled out the letters. Otherwise, you'd simply have to ask. And obviously used in written form such as an e-mail, you'd never know if it were an acronym outside of context. Though a good list can be found here.
A good rule of thumb is that acronyms that might look difficult to sound out are likely spelled instead (FBI, FDA, IBM to name a few).