How do you know when it's time to replace your shoes?

I have a pair of shoes that I only use to workout at home. They've never been outside! So they look pretty pristine from the outside although they are about 3 years old. I don't even see anything that looks like wear and tear on the inside, but I'm not sure if I would know.

I use them about 5 days a week for mostly strength training, HIIT and other high and low impact cardio, so they do get a lot of use. I haven't noticed a change in how I feel over time wearing them, but if it's a very slow change, how would I know until it might be too late (i.e. build up ankle or knee problems due to improper/lack of support)?

I hate to fill up the landfills with perfectly good shoes if they are still operating like they look visually - perfectly fine. But, how will I know when they are not?

As a bonus question, what do you call these kind of shoes depending on your generation/location? When I was little, in Southern California, we called them tennies (for tennis shoes, even if we never played tennis in them!). Now I guess I'd call them athletic shoes (though that sounds super formal) or running shoes (even though my life philosophy is that I only run if someone is chasing me or I need to catch a bus/train). What do you call them?