If your Grandfather passed away twenty or more years before you were born the possibility may be increased. I say this because some spiritual authors have suggested that about 20 years or more will pass between incarnations.
How similar are your personal habits, likes and dislikes to his? Did he keep a journal or diary that you could further compare his life to yours?
Retrograde Planets are said to be indications of where someone "left off in a previous lifetime". If you have a number of the same outer planets Retrograde as were your Grandfather's, that may be evidential also.
Hypnotherapy (Past Life Regressions) can be useful in examining your own past. But don't tell any Hypnotherapist that you think you may be your Grandfather Reincarnated. Keep that to yourself to avoid biasing your Therapist and the outcome of the session. Attempt to review your own past lifetime with a completely open mind as to what you may discover about yourself.