Branches below the graft line should be pruned off, as they are from the rootstock, not from the scion.

Looks like it could use some fertilizer. Where are you located? Can you take a shot of the base of the plant so we can see whether it was grafted? I can't tell from the photo. Also can't tell if any of those vigorous shoots have different shaped leaves, which would definitely indicate they are rootstock. But if you do see a graft line and the shoots are coming from below it, remove them. Otherwise, it's kind of up to you.

Crushed lemon leaves have lemony fragrance. Crush the leaves of the suspect shoots and see if they smell any different.

I didn't know that about the lemon leave fragrance, thanks for the tidbit Dave!