Uber/Lyft will tell you the license plate number of the vehicle which is coming for you. Always check the license plate number before getting into the vehicle. It might feel like a pain in the neck if you are in a State which only has license plates on the back of the vehicles and you have to walk around the car to see it, but you do not want to get in the wrong car.
When you get in the car, the driver should say your name, to confirm that you are the correct passenger.
As PP said, you should request the vehicle when you are at the curb and ready to ride. If you request it early and are not there when the driver arrives, you will be charged a cancellation fee.
You can tip the driver in the app if you like.