It's really not that hard, in most cases.
You say "I think you may be sitting in my seat" while showing your boarding pass.
"What seat is on your boarding pass?"
"Oh, your seat is on the other aisle / 1 row back / last row of the plane"
You can do all this non-confrontationally because it's fair to initially assume it was a mistake (or YOU may be the one making the mistake! ).

The "poacher" may ask if you'd take their seat, and of course you could choose to do that (e.g., if it's across the aisle and you don't really care), or you politely decline ("no, I'd like to stick with my pre-selected seat, thank you").

If they give you any sort of issue, then you can say "Okay, well I'm going to call for the flight attendant" and you press the call button while remaining standing in the aisle. Maybe you try to let people move by you, but if you don't, it makes it easier for the FA to come up from the rear.