Different crown structure, flower structure and flowering times. Northern is tall growing with smaller heads of bigger flowers earlier in the summer (June); in bloom the heads are bunchy enough for it to look kind of like a rhododendron, from some distance. Southern is broad growing with large and open, branching heads of smaller flowers still being out much farther along in summer.

Sounds like 90% of the Catalpas I see around are Northern then.

Most of what you see in the mid-Atlantic states are N catalpa. The best way to tell on older trees is the trunk bark -- flaky, scaly on S catalpa, furrowed (like most other common trees) on N catalpa.