Here is a picture that shows one way to determine proper elbow position at full draw.
The "force draw line" is determined by drawing a line from the drawing elbow to the center of pressure of the bow hand. (see the thread about grip in this forum.) If the force draw line passes through the nock of the arrow, your elbow is in the correct position. You can't look for yourself. Either video, take a picture or have someone watch you.
The force draw line can be used for many alignment issues but this is one of the best uses for it.