Yeah, safest only reliable way is the the ol hydrometer. At least with that thing, you can always tell for certain when fermentation has stopped using the 3 day rule. The only exception to that would if the fermentation has been going very slowly already and temps are also very low. Otherwise fall back on the 3 day rule of thumb.
RE: " My concern was the yeast wasn’t going to make it to the end and I might have to add more to it"
Never heard of yeast not 'lasting' until end of fermentation unless
1) The ABV exceeds the viable limits of the yeast
2) Conditions become toxic to the yeast in someway.
There may be other causes but generally speaking as long as the yeast has sugar and nutrients it just keeps multiplying like mad until the sugar runs out or the Alcohol level rises to the point that the yeast can no longer survive.
As to air leaks during fermentation, I don't worry about them as long as they aren't big enough for fruit flies or other critters to invade. Now at the very end of fermentation, a good seal becomes more important of course and then you should be in a carboy, barrel or some container you can obtain a good seal on easily.
