Some standard advice I hear from sports coaches in the UK:
1. The knee should never extend past the toes, and should ideally be above the ankle.
2. Do not circle the knees - a common exercise in martial arts that puts unnatural strain on the joint.
3. Never lock out joints when doing body conditioning exercises
I go to a weekly class that specialises in lunges and squats. Here is when I am told - I hope it helps:
When doing lunges, you should aim to have 4x 90 degree corners, - front knee is directly above the ankle, front thigh is horizontal, back thigh is vertical, back knee close to the floor. Common errors are a stance that is too small, and moving the hips too far forward whch pushes the front knee out. The focus of the exercise is a vertical, not forward, hip movement. The hips should be balanced _between_ your feet, not over the front one.
Finally, work through your front heel, not toes, which stay light. You should be able to wiggle your front toes.
Do it in front of a mirror and look for those 90 degree angles, with your upper body vertical, balanced between your feet. This will give you maximum benefit from lunges, safely.