How to tell how much a blazer sleeves can be let out (make longer)?

I finally found a 3/2 roll, sack Navy blazer in a local thrift over the weekend. It fits me perfectly except the sleeves need to be lengthened. Looking at the sleeve, it does appear that the sleeves at some point or another had been shortened. Also, based on the length of the blazer and how it fits on me, there is no way the current sleeve length is how the blazer was originally.

How can I determine, while viewing in the store, how much sleeve there is left to be let out? I need probably 1/2 to 3/4 of an inch.

Thanks as Always!

How to tell how much a blazer sleeves can be let out (make longer)?