I'm not knocking fainting jabs or right hands to initiate a left hook, as most have mentioned already. All good solid techniques.
But you could also simply take a quick step with your left foot, transferring some weight onto it. Once you have some weight on the front foot, you have several options to finish the motion. If the opponent stays in front of you, you can throw the lead left hook (it'll be crisper because you took the step to make the angle tighter) or a short/mid-range right hand. If he tries to step back, you can throw a long right hand to follow him. If you don't see any opportunities you like, you can push back off of that left foot and get back to a longer distance. So from that little step, you have so many options, and your opponent doesn't know which one is coming.
Look at videos of Roy Jones. That's how he got away with leading power punches without feinting. But you have to practice pulling the trigger on whatever opportunity your opponent gives you at that particular time, and doing it in milliseconds. Sometimes it'll be a lead left hook, and sometimes it won't.
The best way to throw a lead hook without telegraphing it is to not "plan" to throw it beforehand. Just wait for the opening to present itself.