I've always figured that if you keep your hands moving, jab pumping and throw in feints, you can hit most gym fighters with just about anything.
Once someone has to start guessing what you're going to throw, they start having to deal with avoiding/ blocking your punch AND throwing the proper counter.
If you're worried about being countered, stop throwing in singles.
Some solid jabs should lead to your jab feints being respected. The jab feints will allow you to shift your weight more onto you front foot to put some weight behind that hook.
I try to throw the lead hook off feints after stepping around a bit so, unless I totally whiff it somehow, I can wrap them up if they block or slip it and I can hit them with a straight if it lands solidly.
The forward weight shift and how far down people drop their hand before throwing the punch are the easiest signs to pick up on. The fact that everything about your stance now looks like you're about to try something new is another.
The first two signs will have you leaping onto a right hand, the last one puts people completely on guard so you have no real chance to land it.