Is there any condition saying something like "Unit has attached actor"?
E.g. a unit can only have 1 actor at the same time so I do only want to run the trigger when the unit doesn't have an actor attached, I know it can be done via a unit group named like "units got attached units" but that's a bit annoying as I would have to make many triggers as I'm having like 5-10 different attach-able units.

Actor -> Actor from unit: "Returns the actor for the specified unit. This will return the primary actor that is responsible for governing the behavior of a specific unit, chosen through the Unit parameter."

... I wanted to find out the attached actor, not the primary one.

Then why not use this method? Add all the "attachable" units to 1 unit group so you can manipulate them easily.

.... someone else?

Not that I know of but assuming your attaching them via triggers you could use the same trigger to set a variable saying that its attached.

Again, do it with dummy abilities or behaviors instead if you don't get people with smart ideas.