[DCP-o-matic] Color Space Rec 709 or sRGB

Hello! I think this question has been asked before, but I am still not comfortable with it. I understand the difference between these two color spaces, but how do I know what to choose when working with DCP o Matic? How can I check the color space of a video file? And what do I do if I do not know it? When working with HD Material, is Rec 709 the common color space? When I export from Premiere to Avi uncompressed, TIFF sequence or H264, how do I know what the color space is? Any answer is appreciated! Thanks Andi