The usual hallmarks of a past life memory, as opposed to a day dream, imagination or fantasy, is the way they make us feel - usually some kind of strong emotion. Happiness, sadness, fear, anger - it's really hard to make yourself feel an emotion out of nowhere. Actors have a lot of training to be able to make themselves cry for instance, but most often they are using a real experience from their own life to get to the emotion they are looking for.
The level of detail that is usually involved is hard to just 'make up'; the way we just know what is going on, what we are thinking, who all the people are and so on is something that is hard to fake - even to yourself. Often the number of senses involved and all interacting together is unlike something we can just dream up - there is hot, cold, smells, wind, sore feet, sweat, dirt, buttons, sounds, etc. Most people just don't have that good an imagination. I know I don't. If I tried to make up something as detailed, and historically accurate, with all the background information I seem to have about what was going on at the time in some foreign country centuries ago, I just couldn't do it.
Just trust your instincts about it. You will know deep down if you are 'making it up' or remembering. It feels different.