I have been lucky so far. A whole storage unit full of 80s motorcycles, two 80s trucks, and a 1963 outboard. No problems to report yet. I have removed float bowls on most to look for stains or accumulation but have found none. It may be that my distributor is mixing ethanol at the minimum 2% required by state law. Others say they do have the full 10%. I use Stabil also which would be expected to help. If I start to see any problem I am going to switch to 100LL for the bikes and outboard but that will be expensive. Hopefully manufacturers will come out with retrofit kits for some of the popular older carbs such as the Walbro in my Elgin.
You can get testers, and most times the ratio has to be posted on the pump. We’re telling all our customers to make sure they don’t leave gas in their equipment for any length of time. Stabil helps some, but the ethanol is literally eating the fuel lines, carbs, etc and gumming up the carbs. Nasty stuff. Leaves a white residue.