How to check if a field exists on a section
I think you're running in to a caching issues here or you're not testing with devMode turned on perhaps.

I've just tested here and if I remove a field and reload an entry in devMode I immediately get an exception e.g.:

Craft\Commerce_ProductModel and its behaviors do not have a method or closure named "unimformityMeasurement". 

Actually, even with devMode off I get a 500 server error with the same information, so I am thinking that you're very likely hitting a cached result that is preventing you from  seeing these errors.

(In my experience you are correct the data is still in the database though).