A lot of it is timing, technique and pactise. If you lead early or late, it takes a lot more force to get the lady to follow. It will also be a lot more awkward for the lady.
Technique is also critical. With proper technique, every dance has points were the ladies momentum is stopped/changed and she can be lead smoothly without much effort. Example the fall in Waltz, stops both dancers momentum and makes it easy to change the ladies direction. Without the rise and fall, it is much more difficult to lead Waltz smoothly.
In technique also lead correctly, if the frame is communicating what the body is doing, make sure the frame is not moving.
Key way to judge yourself, is if the lady is following, moving smoothly and not losing her balance. Focus on the lady and adjust your timing and the way you lead a move, so the lady can move smoothly.
Remember the lead is a communication to the lady, asking her to do something. It is not making her do something.
Also always lead smoothly and gently. Even in moves were the lead takes more energy, you still want to do it smoothly and gently.
Practise, for me it takes some practise to find the point at which to execute the lead and how much energy to exert.
Be aware of the ladies capabilities, do not lead things she can not do smoothly, even with a perfect lead.