A really simple test is to measure voltage after it has been sitting a day or so, and it should be at least 12.8 or more. The best test is to go to a battery place and they can load test it. Three weeks is not really long enough for the petrol to dry out from the float bowls, which can cause problems on the ST1100 as the vacuum tap will not let fuel through until the bike starts and there is vacuum. But if the bike turns over reasonably, a well running ST1100 should start first go.
I have stored mine over the last year, and put in a product called Stabil that supposedly keeps the fuel fresh while in storage. You dose it up, run the bike for a bit to get the fuel through, then let it sit. Mine starts first go every few months.