You really need to be able to take gravity measurements. Airlocks don't really indicate anything. Even with a perfect seal, afetr main fermnet is done, it'll still be going for awhile. The ony way to tell for sure is to have it go for 2-3 days without changing. If you just left it alone for 3 weeks, it should be complete and gives time for yeast to scrub off.
Michael raises a good point in that it's possible for gravity to remain the same for a few days, but have a stalled ferment. You should be able to glean enough data from the manufacturer, and other users on what your projected FG should be (or range). Try searching that yeast here, or user reviews on the White Labs site.