How the BB and crank connects is the first place to look. Most likely it will be square taper, (which is a square bar with a taper to it). If its not, then it could be one of a few other things like ISIS, cottered, or some other new stuff I'm not familiar with. If the bike is very old, it may have a single crank bottom bracket, where one piece of metal goes from one pedal, through the bb, chainrings, and finally to the other pedal.
If it is a square taper, then look at how it connects to the frame. If it has a series of recessed notches in a ring then it is probably a sealed bb. If there is a lock ring, with notches on the outside of it then its bearings are probably accessible with the proper tools.