However you expose it, it always can be faked IMO...

Um, the solution is to NOT expose it

If its private data, only the server is supposed to modify, the solution is to only let the server modify it

I think you need to explain the architecture of your application, draw a plain ASCII diagram or flowchart

Think of the server as a bartender, who needs to check id, serve drinks, make change ... only bartender touches the cash register, customers always have to go through bartender, and he knows how to spot fake currency, fake ids, how much vodka is left ... bartender is gatekeeper

See The Architecture of Open Source Applications