If requiring a mirror...assuming it's for BB'ing general progress or posing.
A three way full length mirror. Also a setup to view the total back, with that three way mirror. Using direct lighting or overhead sunlight can be very telling as regards finding flaws in physical structure. Flat lighting is very misleading and can give a washed out appearance. Set up a video camera to check progress from time to time.
If using a tape measure, use a narrow steel tape. Try taking measurement first thing in the morning rather than later in the day, when daily movement can encourage a natural pump to the muscles. . Taking measurement during or after a working will never gave a true reading.
If wanting to be truly honest, than don't workout for a couple of days and than take measurement first thing in the morning. You might be shocked.
If shirts are tighter around the shoulder girdle and upper back or if jeans/pants starting to feel snug around the legs/calf's, that is a good clue your making progress. Hopefully it is increased muscle mass, rather than fat. See too many guys who some how equate fat as muscle gain. Only fooling themselves and no one else. If waist measurements stay the same or by some miracle become looser, you are on the right track in training.
Strength gain progress:
That is very obvious. Don't need any commits from me. Though I have met a few still confused by the simple training stuff.. Never could understand that.
Good Luck.