There's quite a few ways to confirm a user ID (uid) in Unix.
This is probably one of the easiest ways to find out a uid of a particular user in your system:
# id -u greys 500
The most common way of using the id command is even simpler, and it gives you all the information about a user you may need:
# id greys uid=500(greys) gid=500(greys) groups=500(greys)
This not only shows you the user id (uid), but also confirms user's group id (gid) and all the rest Unix groups a user belongs to.
Another way to get information about a user (including uid) is to use the getent command:
# getent passwd greys greys:x:500:500:Gleb Reys:/home/greys:/bin/bash
In this example, greys is my username, 500 is the uid, and the second 500 in this line is my Unix group id (gid).
If you know that the user you're looking at is a local user (it is created on the local Unix system of yours), you can grep for it in /etc/passwd file directly:
$ grep greys /etc/passwd