Docs, could a doctor tell you have asthma just from a checkup?

Depends: If your history is suggestive your doctor could pursue some lung function testing that might confirm your suspicion. If s/he could not trigger some evidence of the disease with provocative testing, s/he might need you to return for an evaluation when/if you had some symptoms. With no suggestive history or findings on testing, it would not be evident on a physical alone.