If the spindle was symmetric (many are not), then theoretically what you're saying is possible, but measurement of chainline and the relationship of chainline to spindle isn't standardized. All in all, it's much easier simply to take off the crank arms and measure the spindle length. Even this doesn't mean you're totally out of the woods if your spindle isn't symmetric. There are tables that will help you convert a given assymetic spindle length to a symmetric equivalent that will give you the same chainline spec.

Some reading:

Chainline Concepts | Park Tool
Sheldon Brown's Bottom Bracket Size Database

If you give the gurus here more on what you're replacing and what you're trying to replace it with, you'll get better guidance.

- Mark