How can I confirm french ancestry?

Question:

How can I confirm french ancestry?

While I was going through some archives I discovered my grandfather[deceased] and his brothers had french middle names and rang up a close relative of his.

She told me my grandfather was of french descent that our surname was actually De Fontaine,not the dutch [van rxxxx] surname I have always had,she said my g.g grandfather had changed it.

How do I go about confirming IF this is true??

Best Answer - Chosen by Asker: do you run away from the slightest threat. If you do you will know you have french blood.