[Admin]

if ($usersettings=$DB_site->query_first("SELECT * FROM weather_usersettings WHERE userid=$bbuserinfo[userid]")) {
	// user is there
} else {
	// user is not there
}

/* ### OR ### */

$usersettings=$DB_site->query_first("SELECT * FROM weather_usersettings WHERE userid=$bbuserinfo[userid]")
if ($DB_site->num_rows()>0) {
	// user is there
} else {
	// user is not there
}

Very simple.