check if user exists
132697
use EntityFieldQuery to find user based on profile field value
96308
Check if Cookie Exists
11663
Checking a list if a certain string exist in visualforce page
114938
Check if email exist php
76435
ValidateCredentials returns true for unknown user?
19994
What's the best way to check if person accounts are enabled via Apex Code?
5473
Check if user is logged in using JQuery
2818
How can I create an infinite login loop for a specific user?
125402
Run Trigger As A Specific User (or Profile)?
69750
MySQL check if a table exists without throwing an exception
134025
How to check if an array value exists?
157008
Check if counter exists
143370
Evaluate Current User for Flow - 15 - 18 digit User ID
84734
Does there exist a user with this role?
84994
Make searches for user:xxx work for deleted users as well
144101
Programmatically create a user account
86214
if(!isset($_POST["user"]) ignored and returns Undefined Index