Start here: https://swspaceclub.com/wbobafett/coo-guide-3-0/anh/darth-vader/#a
Group your figures into those various mould families. Check the length of the saber arm 'tube' - usually an easy way to tell 'moulds' apart. When all else fails send M4K3R1 ( Oscar) a message.
Have you looked here? http://www.imperialgunneryforum.com/t223-darth-vader-figure-variants-thread