The easiest way to tell the sealant's composition is heating it heat gun or very hot hair drier and see if it get gummy. If it does, it is NOT urethane. Urethane does remain flexible after cured but does fully cure while buytl never cures and can be re-flowed using heat.

Removing buytl can be done with heat and lots of rags but it is messy. Mechanically scraping it with bunch of old credit cards or disposable plastic putty knives will get most of it off (a little heat to soften it up REALLY makes it go easier). There are also some very nasty chemicals that can be used to assist in removing the residue (I think Acetone was popular) but I seem to remember that diesel/kerosene can also be used (do not take my fuzzy memory as gospel, though).

Other than mechanical scraping, I know if no method for removing Urethane. On the plus side, it will come off in a long string if you can get a razor between the metal and urethane and follow as you pull.

I have not experience with Urethane sealant for installing windsheilds but I will say that if you are ever going to remove the glass again yourself, buytl is the way to go because of the reason I listed above.