Notice the emotions you feel – if they are strong (sometimes very strong if it is a death scene or another emotional scene) you have definitely gotten to a past life memory.
Brian Weiss says that even if you feel that it is made-up or it feels like imagination, it all comes from you and can be beneficial and healing to you. Healing is the point of a past life regression, so if you think it is imagination and you healed from the experience, then what does it matter. Some people, who do not believe in past lives, do regressions under the belief that it is all imagination, yet can still reap the cathartic benefits.
It may sometimes feel as though it is imagination, but often this is often your ego butting in or your rational mind trying to stop the memory.
I have personally had a few experiences that seemed a little like imagination at first. (For a past life regression to go really well for you, you must be open to it and to TRUST whatever information comes to you.) But, as the regression progressed, it was definitely not imagination.
Also, if you do regressions more than once, then the next time (or times afterwards) you remember this same lifetime, the story does not change, but more details from that life are remembered. This is also another way to know it is not your imagination.
For me, I get tears in my eyes when something tragic is starting to be remembered and often cry (sometimes a lot) when a memory like that is coming through. It can be that emotional. It can be tears of joy as well, like when recognizing someone from this life in one of your past lives or for me, hearing music I loved.