One of the things coaches stress is that you should be aggressive against deep serves. (A short serve means that, given a chance, the ball would bounce twice on your side; a long serve would only bounce once.) Against a short serve, you can rush and angle the server by taking the ball quick off the bounce, plus you can drop the ball short. But against a deep serve, you can't do this, so a passive return gives the server an easy attack. If the server has a weak attack, that might be okay, but in most cases, if you don't attack deep serves, you put yourself at a disadvantage as the server gets an easy attack.
But to attack the deep serves you first have to recognize that the serve is long. How do you do this?
Imagine an outfielder in baseball running down a fly ball. (Or any other sport that involves judging the trajectory of a thrown or hit ball.) He doesn't do mathematical calculations to judge where the ball will drop. He simply watches the ball as it rises, and from that, with experience, he learns to judge the arc the ball will take. And so, after time, a good outfielder can immediately run to almost exactly where the ball will drop.
It's the same thing in table tennis - not just in judging whether a serve is long, but on ANY shot. With experience, you learn to judge, as the ball is leaving the opponent's paddle, where it will go. (Advanced players take this to another level and often know where the ball is going before contact, by watching the ball and racket as they approach each other.) When an opponent serves, it's the same thing. Watch his paddle, and as the ball bounces off of it, you should be able, with practice, to almost instantly judge its trajectory. If you can do that, you'll immediately know if it's long or short.
More specifically, you'll see how fast the ball comes off the paddle, how downward it travels (which lets you know high it will bounce, which is part of the trajectory), as well as the spin. (Topspin will make it bounce out at you, and so usually goes long, while backspin slows it down, and is more likely to pull the serve short.) Advanced players can judge the depth reflexively as the ball is leaving the paddle or sooner. With practice, you should be able to do so before the ball bounces on the server's side of the table.
You do have to judge it quickly as it takes time to set up an attack. So how can you practice this? Get a coach or practice partner and have them serve to you! Ideally, have them do "half-long" serves, where the second bounce, given the chance, would either bounce very close to your end-line, or just off it. You get to judge which it is. It can be difficult to tell if a ball is going to go one inch long or short, but you should be able to judge it so that if the ball goes six inches off, you always loop it. And then four inches, and so on. Top players are masters of judging this to within an inch or so, and instantly jumping on serves that go too long.