Do you mean you want to replace your current solid axle square taper BB with a new hollow axle square taper BB?
Or do you mean you want to replace your current square taper BB with a modern hollow spindle crankset, like a Shimano Hollowtech unit?

If you mean a new hollow axle square taper unit, take the crank arms off and use calipers to measure the width between the ends of the tapers.

If you mean a new hollow style non-square taper crankset, you just need to know the BB shell width (68mm or 73mm).