Make sure your seat is centered over the saddle and your body is bending or folding over at the hip. This will force your legs into the correct position because they are truly your base of support. Get yourself steady in this position and then slowly bring your shoulders up and your seat into the saddle without budging your legs.
Glance down over your knee without moving your head or anything else in your position. If you can see your toe in front of your knee, then your lower leg is too far forward. That's one quick check for you if you don't have an instructor on the ground! :)