The process of checking a user ID in Linux is relatively straightforward. In order to check the user ID, you must first open up a terminal window. Once you have done so, type in the command ‘id’ followed by the username that you want to check. This will display the user's information including their UID (user identification number). Alternatively, you can use the ‘cat /etc/passwd’ command which will list all users and their associated UIDs. Additionally, if you are using Ubuntu or another Debian-based distribution of Linux, then typing in ‘getent passwd ’ will allow you to view details about a particular user such as their home directory and shell preferences.