Add-ons and redos are often cosmetic. Look for places without makeup. Closets, Cupboards, attics, crawlspaces. In brick buildings observe closers. Floor plans are the best way to see the relationships of disparate evidence.
Structural members are less likely to be changed than trim.
Buildings rot from the bottom up. Look for original framing higher in the walls. The archaeological metaphor. The lower layers of a site must have been deposited first. Try this slide to test your powers of deduction.