Is there an easy way to figure out what size BB I need to replace one already on my bike?

Also, what is the deal with the new edition of BB's? I have been looking into getting
a new crankset eaither a truvativ stylo team a RF evolve XC and shimano LX. At this point I am leaning towards the LX. Bsed on the BB reviews for the RF and truvativ
why would I what to invest in them? I would like to go for the XT but Jenson has them
backordered and I wonder whether they will ever get more since it is last years model.
any thought