Identifying frame boundaries in RTP packets

Hi,
I have few doubts regarding frame boundaries in RTP packets.
First, If the marker bit is set, does it say that a new frame has begun(this is what I understand from RFC 3551)?
Second, According to what I read a frame starts with I-frame followed by P, B frames. Now, which field indicates this? And is the I frame has the marker bit set?
Third, If I need to find the start and end of a frame, would the check for marker bit suffice?

Thanks!