Check if items can be used in main hand

I tried looking to see if there was a topic for this, but I couldn't find one. I was wondering if there was a way to detect if the player can use the item in the main hand. That way, I could have some of the custom items I create usable in the offhand (just like vanilla dual wielding). 

Now, this might just be solvable with tags, but it would need conditions. For example, if you're holding a snowball in your main hand, and a custom item in your offhand (that is also usable) I want it to just use the snowball, and not the item in the offhand. 

I know this can easily be solved by detecting if there is nothing in the offhand, but the item I'm creating would be very useful in combat (so I want it to be able to be used when the player is holding something in their main hand).

Like I said, this might already exist in Mcreator but I haven't found it, and if it does exist, I need some help finding it.