Some people can tell you everything about themselves by just their appearance, the way they walk, talk, and breathe. Everything is just a little spoiler of how they actually are as a person. When you work in customer service, you learn to discover character traits of customers before they even open their mouths to talk to you. Is something about them that can be almost imperceptible to anyone else that will tell you everything you need to know about them. Whether they are going to scream and mistreat you, whether they will be extremely cooperative and apologetic about everything, or if they will just remain calm during the whole interaction.
These little things that make employees know their customers beforehand can be different for each worker. Some people notice the way they walk. If it's with confidence, then they will probably be entitled, but if they walk in a reserved manner, then they will try to avoid any kind of confrontation. It can also be their perfume. According to the smell, they may determine if they reserved the biggest penthouse or just a simpler suite room. Every angle and detail that you can sense with your senses will tell you beforehand everything you need to know. You just have to learn to interpret these signs.
These things aren't infallible. They can definitely give you the wrong impression about a customer, which can actually cause even bigger issues. So it's best to always keep that judgment you make of them to yourself and act as you would with any other customer. Pretend as if you haven't noticed anything about them. They are just assumptions, but they can be extremely useful to deescalate complicate situations. Just be wise about them and don't just assume you know these people just because of the traits you discovered. If you want to know even more about these little imperceptible traits, I gathered below a thread about which ones most employees notice.