Hi and welcome!
You should be fine.
I didn't know that B*Men had been discontinued, but if it was, it must have been recently, so the bottle you saw in the shop won't be very old. A bottle stored in its box will keep fresh for years. I would be more wary if we talked about a bottle 40-50 years old.
Lots of us buy discontinued fragrances, sometimes you end up buying a bottle where the juice is slightly off, other times it's completely ruined, that's the "thrill and danger" of loving vintage perfumes But now I'm talking about fragrances significantly older than B*MEN.
There is no easy way to find out when the bottle was manufactured. The box and/or bottle will have a batch code, usually unintelligible to anyone outside of the factory, but if you manage to crack it you will at least know the year when it was made. On some products you can use this nifty date-calculator, but I don't think it will work with Mugler: http://cosmeticswizard.net/index.html
Still, the odds are in you favour, I think you should get it.