The only thing about B12 now, in contrast to B12 back around, say, 1976, is that every supplement I have looked at only doles it out in MICROgrams instead of milligrams.
I started bodybuilding back in 1970, obviously well before the current craze. I used a balanced B vitamin, in which EVERYthing was a 50 milligram (or 100 milligram) dosage, so there was sufficient strength in it. I was pretty disgusted when I bought it earlier this year and saw that nearly every manufacturer -- including the company that I used back in 1976 -- only provides them in doses 1/1000 of what they used to. Another good one is B15, which is, unfortunately, not easily found in stores. It is called calcium pangamate. I used to use balanced B, 1200 units of E, and 100 mg of B15 before working out. I found working out easy (but then, I was around 20 as well!) and got results very quickly.
B12 is an excellent energy vitamin, and I think doing a panel, as the other poster noted, is an excellent idea. It is not, however, something doctors routinely think to recommend. Most tests results simply come back "within the 'normal' range" which is silly, as every body is different and some need more of a mineral or vitamin and some need less.