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).
If you look at the third definition of acronym, it directly addresses your question. BBC is an acronym, regardless of whether it is pronounced as letters or as a word. RAM (random access memory) is an acronym whether it is pronounced ram or are-aye-em.
SUPPLEMENT
There is a controversy as to whether initialisms are a subset of acronyms or a distinct class of terms.
Several dictionaries seem to restrict acronyms to those shortened forms that can be pronounced as a word. See, e.g., Oxford Online, Collins, and American Heritage.
Others include initialisms as a form of acronym. See, e.g., the OED, Merriam-Webster and Websters New World. Wikipedia has an article on acrnoyms which notes a difference of opinion as to whether initialisms should be included, and then proceeds within the article to include them. It also notes that there are a number of forms that do not neatly fit into either category, such as JPEG (pronounced jay-peg) and AAA (usually pronounced Triple A).
There is also the interesting evolution of an acronym found in the abbreviation for Standard Oil. It began with SO, a clear initialism, which them morphed to ESSO, a classic acronym. (The company has since become EXXON which does not seem to stand for anthing.
Best Answer
An acronym (as the term is commonly used) is a term that is formed from the initial letters of some longer name and is pronounced differently than the expanded form. Examples of acronyms as the term is commonly used:
An abbreviation is written differently from the expanded form but is pronounced the same. Examples:
Whether the form is pronounced as a “word” or as letter names or as some combination of the two is a red herring. The key difference is whether the abbreviated form is pronounced differently from the expanded form.