How to know if chicken is properly cooked without a food thermometer? [duplicate]

The only alternative to a thermometer is long experience. The longer you keep cooking chicken while paying very close attention to it, and then take a few minutes to evaluate the texture of the cooked chicken instead of wolfing it down, the more you get an intuitive feel for when it is exactly ready.

After some years of doing this regularly, you will get it right without a thermometer in probably 95% of the cases, with time even more.

You will surely see advice claiming that there are other methods. For example, it may describe the feeling of the chicken breast when you press on it. The problem is: when this advice is right (and it isn't always), then it is the same as saying "by long experience". Sure, the tactile feeling of exactly right cooked chicken is different from undercooked and from overcooked. But the difference is too specific to be transmitted by words. You first have to spend a long time comparing the feeling, by cooking many portions of chicken, before you can tell when it is right.

Rumtcho's answer is the way to go, you know this through long experience - but 'bite-sized' pieces should be about 3 minutes total as a 'keep it moving' stir-fry, not 7 minutes each side. That's going to be completely overcooked.
After comments - I'd always cook bite-sized or thin sliced chicken fast & hot, rather than slow & cool. By the time you get a hint of colour on the outside, the inside should be about done.

Whoever said "it's still stringy", implying "still undercooked" had no idea what they were saying.
Slightly undercooked chicken, it you cut or pull it between forks [don't eat it] will not be stringy, it will be very tender, like a rare steak compared to a well-done one. Over-cooked chicken will go stringy, then eventually completely dry, chewy & bland, just like an over-cooked steak. Your aim is to hit a point between the two, because you can eat rare steak, but you cannot eat rare chicken.

It's a tough job to get a thermometer in the middle of a bite-sized piece & the degree of accuracy will be far too broad to be reliable.

To test for doneness, smash the biggest piece on a plate with a fork, or tear it with two, to get to the centre. If it's got even the vaguest hint of pink in the middle, that one needs to go back in, the smaller ones are done. If you're still unsure, test the next largest piece. It will keep cooking through as it's served, so if you wait too long, hesitating about safety, you'll over-cook it every time.

How meat "feels" (stringy/pudding?) in the mouth can be an indicator of being under/overcooked, but unless you already know what the result should be then it is a poor indicator so other methods should be used.

In your case (of shallow frying chicken), you can poke a metal skewer into the thickest part of the meat, especially close to the bone (if it has bone in) and then press the meat, if the skewer comes out very hot to the touch, and juice run out clear then this is a good indicator that it is cooked, if it is pink then it is likely to be undercooked.

I will not say the cooked meat should be white because unlike breast meat, cooked leg meat will generally be more fawn coloured. But as long as the meat is not pink, then it should be ok.

I would highly recommend to get a thermometer, and test the meat so that you can learn when it is cooked while using your chosen cooking methods. You can then go without the thermometer once you have a good sense of the expected colours of cooked meat and juices.