Tha tapers are all more or less standard as long as it's a regular ISO taper and not a JIS. JIS being rather rare over here. So the taper SHOULD seat in the cranks the same to within a mm or so.
So if you measure your present spindle length and work with measurements of both your crank arm to stay gap and chainline measurements some simple math will get your selection to within a couple of mm's. Keep in mind that the length is overall and you'll be measuring the ONE side so allow for double the difference when selcting the length. If you're still not trusting yourself then as mentioned buy a cheapie cartridge BB or get a discarded one of that length from one of the LBS's in your area and fit it to your bike just for sizing. Note that unless you're pretty careful that measuring the length of the current BB spindle would most likely require removing the crank arms to be sure you get the distance correct. Or do up a little diagram and then use calipers to measure from the outside faces of the arms, then after removing the crank bolts measure how deep from that same spot the end of the spindle is and do some more math. But if hte area around the crank bolts are sloping or have other fancy shaps I'd suggest to be sure that you just remove the arms and measure the actual spindle.