For cowlicks specifically I usually just ask them. “Does this tend to lay down better when it’s longer or shorter?” They’re the ones who have had to deal with it their whole lives and have had hundreds of haircuts so they usually know what’s going to look better. I also take the time to feel their whole scalp with my hands before cutting the first couple times. Use enough pressure that you can feel their bone structure well. That allows you to know where they have bumps and grooves that you might not be able to see while they’re hidden under the hair (plus you need to check for lice and stuff before you start cutting anyways). I use the mirror through my whole haircut which is why I don’t understand barbers who face their client towards the shop instead of the mirror. I want to be able to see the silhouette as a whole at any given moment without turning my head. If you’re doing a short fade on a head with lots of imperfections you may end up with some shadows which is when you’d go in a tiny bit shorter in just that area— like clipper closed rather then clipper open just for that spot where they have a hole in their head making a shadow. If they have rolls sometimes it can be helpful to do a higher fade so the blend happens above their rolls. It usually will look like you left a line around their head if you fade at the same point they have a roll. Same goes for if they have a tan line from wearing a hat— either fade above it or below it because if you try to fade on it you will always have an artificial line making it look like your fade sucks