If you don't have the courage to add tail weight right off, observe what the thing does when the engine cuts (assuming you don't have the motor offset up or down):
If it tends to dive, and seems to speed up, it's nose-heavy. If it stays pretty level then it's about right. If it "balloons" and slows down, then it's tail-heavy.
I'd try Randy's suggestion if it does either of the first two.