Is it easy to see when my tire pressure is low?

No it isn't.  I've had plenty of tires that looked fine, but when I checked them with the gauge, they were under.  For years I had one of those old style tire pressure gauges, where a plastic rod with numbers on it shoots out, then I got an electronic one on ebay for $7, so much better.

Only if you're observant and have a trained eye.  Many people don't see a flat tire, because they don't pay attention to their surroundings.

Difficult to tell by looking esp with low profile tyres, you certainly couldn’t tell between correct and 10 psi lower which would be dangerous, hence new cars have tyre pressure monitor systems. However, they are unreliable , so just buy a handheld gauge, they’re pretty cheap. 

The only way to make sure your tires have the correct pressures is to use a tire gauge. They cost under $5.00! If you can see the tire is low it's low enough to cause tire damage.

It is if you check the tires with a tire pressure gauge.

Yes. Reading a tire pressure gauge is not difficult.

No. Modern tires don't show low pressure until they are dangerously low.

But a tire gauge ----------------- USE IT ! ! 

Yes if you have one of those cars that have TPMS (tire pressure monitoring system). It turns on when the tire pressure is too low. It’s one of the new safety measures put in place to reduce fatalities.