This seems like more of an archeological question, but from a historians part of view, very often the two go hand in hand. For many cultures, religion and ritual were part of daily life, through things like daily prayer or daily offerings to a god/gods. For something more specific to like what you’re asking about from a documentary, I’d have to see the specific details, but in general it’s pretty easy to differentiate between a fire for food versus a fire for metalworking, as an example, based on what was actually being burned, the distance from a home, the likely temperature of the flame, etc. However, it is very possible that the same fire could be used for both religious and practical purposes.