it will take a little time ,but after a while you'll know the difference. If it was wear other parts of devices would be worn also. The answer to your last question : you should be able to tell the difference between wear and poor strike. the 1918-D is a gem but the center band is not split because of wear. doesn't have to have FSB even if it's graded MS-67 Or 64,or 68 it's an added bonus that would give your coin much more value. So a 1942-D MS-66 fsb would be worth considerably less than the MS-65 45-P fsb. Like I said it's a true rarity for the 1945-P to show FSB. hopefully Coop will come in and tell you about more Merc.
other dates sought after are the obvious 1916-D, 1921-P, 1921-D.
It is a rarity, I far as I know the reverse bands just weren't struck up at the Phily mint in 1945. I would have to say the most sought after common date Merc. Lets see if I can clarify some of your questions.