How can I check if on specific plugin generated page or child

If you’re trying to catch the pattern ‘/forums/user’, you could use PHP’s stringpos function. Something like this should capture what you’re looking for:

$url_pattern = "/forums/user";
$requested_uri = $_SERVER["REQUEST_URI"];

if(strpos($requested_uri, $url_pattern) == 0){
    //Your code goes here
}

Make sure if you have “enforce trailing slashes” set that you use “/forums/user/” as your matching pattern instead. This code isn’t tested but should work.