When I said it is hard to know when God is being literal and when he is being allegorical, I should add that I believe the Holy Spirit (in prayer) brings a lot of clarity to each believer re: such confusion.
The Bible employs metaphors, similes, hyperbole etc as figures of speech. They are interpreted in the ways that the figures of speech are commonly understood. There are cases where symbolism is used , but often the text will explain the symbol in subsequent verses as in book of Revelation. If not, there are often clues in parallel passages.
The problem with allegorical interpretations is that they are often based on the imagination of the interpreter and are wholly subjective. This allows the interpreter to change the meaning of a passage to fit another narrative.
We see this in the allegorization of the Genesis narrative. Many cannot believe in 6 literal days of creation and they seek to redefine the meaning of a day. This is despite the fact that each verse also adds the detail of morning and evening associated with each day, further defining the day as 24 hours.
Context is the key to understanding.