$gameParty.leader().hasSkill(id)
where id is the skill id with no leading zeros.
You will learn a lot by checking the rpg_objects.js file. Search for Game_Interpreter.prototype and then anything after that is what sits behind the event commands. So if you can do something with an event command, search for the command name in that file, and you will see the code that it runs. For example, there is a check in the conditional branch to see if a certain actor has learned a certain skill. So you find the Conditional Branch code, then read through it to see where it checks for the actor having the skill. You don't want to use a specific actor, but the party leader, so where the script has
actor you just substitute
$gameParty.leader() instead.
You ask a lot of questions where I go to that part of the script to find the answer, so I'm sure you'll gain a lot by taking a look through it - doesn't matter if you don't understand it all - just seeing what's there, and going back to it each time, will help you understand it bit by bit