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I had the same issue - got tired of making frequent trips to the shop. I tried everything mentioned above, and honestly, visually peeking through the disc gap is basically guesswork. But there's one trick nobody really talks about: listen to the sound when you brake. When the pads wear down completely, manufacturers intentionally glue a metal wear indicator into them, which makes a high-pitched squeal or screech when you brake. The sound is very distinctive - you'd only confuse it with moisture-related squeaking, but in different weather conditions it becomes clear.
The main thing though: if you're going to check without taking it apart, look at the brake fluid. The transparent reservoir on the engine has min and max marks - when the pads are badly worn, the fluid level drops (the piston in the cylinder travels deeper). It's not a direct indicator, but it's a decent indirect hint. The catch here is: the more often you check the fluid level, the more often you need to replace it with fresh - it oxidizes quickly in air.
But the guys are right - this is all band-aids. The only truly reliable way to know about wear is by removing the wheel. Takes half an hour, but then you'll know for sure and won't rush unnecessarily to the service center.
In reality, you can't reliably check the wear without removing the wheel - all methods like looking through the disk gap or a visual "rough estimate" give you errors. If you really don't want to take it off, you can peek at the pad thickness through the gap between the disk and tire (you should be able to see at least a few millimeters of friction material), but that's just surface-level stuff.
The right way is to remove the wheel and inspect the pads from all sides, and while you're at it, check the disk for cracks and scoring. It'll take about 15-20 minutes, but then you'll know for sure what condition they're in. If you're not confident in your own abilities, it's easier to just drive to a service station and ask them to take a look.
The second answer cuts off, but to what's already been said you can add: if the car is under warranty, many dealers check brake pads for free during servicing and will give you a definitive answer. The sound when braking can tell you something too - a squealing wear sensor doesn't go off for nothing, though you can't rely on it alone. But yeah, for an accurate diagnosis you're not going to get around lifting the wheel anyway.
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