Is the wire gauge on my existing NEMA 14-50 outlet actually rated for a 50A EV charger, or do I need an electrician to check before I plug in?

Turn off the breaker, pull the outlet out of the box and read the cable jacket. It'll say 6/3 or 8/3 or whatever it is. Ten minute job. If you end up calling someone don't tell them it's for an EV

The breaker handle tells you the breaker size, but not the wire. You have to actually see the cable, either at the outlet box or where it comes into the panel. 6 gauge copper is fine for 40A continuous. If it's 8 gauge I wouldn't run 40A on it. What charger is it? A lot of the portable ones that plug into a 14-50 top out at 32A anyway, so you might not even be pulling 40.

Agree on not mentioning the EV. I got quotes for the exact same 14-50 once as "EV charger" and once as "welder." The welder ones were half the price. Same wire, same outlet. But if it really was a welder circuit I'd look at the wire before trusting the 50 on the breaker. People do all kinds of things for welders.

It's whatever came with the car, I think it's adjustable but I haven't dug into the app yet. Went and looked last night and the garage actually has its own little subpanel, which I didn't even know about. The 14-50 breaker in there says 50 but I don't know what feeds the subpanel itself. Didn't pull the outlet, the box is kind of jammed behind a shelf the old owner bolted to the wall

Honestly if you're this nervous about it just pay someone... I paid $400 for my electrician plus the JuiceBox 40 and I've been charging on that circuit pretty much every night for over 4 years and 120k miles. Never think about it. Peace of mind is worth something! And a subpanel you didn't know about is exactly the kind of thing I'd want someone who does this every day to look at. Could be totally fine....could be a 60A feed with a dryer on it too. No way to know from the outside.

Mine was $85 on a 200A panel. It shouldn't cost much to have someone look at what's already there

The subpanel changes things a bit. There are really two circuits to look at now. One is the 14-50 branch circuit, which is the wire from the subpanel to the outlet. The other is the feeder, the wire from your main panel out to the subpanel. The 50 on the 14-50 breaker only tells you what's protecting the first one. Go to the main panel and find the 2-pole breaker that feeds the garage. It's usually labeled something like "garage" or "sub", and if it's not labeled, it's the one whose number doesn't match anything else in the house. If that breaker says 60, you have 60A for everything in the garage combined. That's workable for 40A continuous, but not if there's also a compressor, a freezer, a space heater, and the welder the last owner probably had. If it says 100, you're likely fine on the feeder side and it comes back to the branch wire. I'd also pull that shelf off the wall at some point anyway. Nobody should need to move furniture to get at a receptacle that's carrying 40A for hours every night. A 14-50 can be perfectly to code and still be the part that fails, and when it does it's usually at the plug, where you can't see it if it's behind a shelf.