There’s sort of an indication. It’s the kind of thing that people love to claim “only stupid people miss” but smarter people know how you can miss obvious things when you are moving fast.
The Casual PvP’s Team Selection screen has the difficulty selector on it. Ranked PvP’s screen does not. Usually I only look at my team and sometimes scout the other team, but more commonly I’m reflexively jamming my finger on the button to start the match.
So just about every time I go to casual PvP, I have to quit my first battle because I realize too late that I forgot to switch the difficulty back to Normal. One time I went 4 or 5 battles before I started asking, “Jeez, why am I losing so badly?” It’d be even more annoying if there were an extra screen to confirm, so it’s one of those things you have to learn the hard way and hope you remember.