WHM -> Server Status -> CPU/Memory/Mysql Usage
Or you can check the MySQL usage using the command "mysqladmin status" and "mysqladmin processlist" using SSH.
root@host [~]# mysqladmin status
Uptime: 7947 Threads: 1 Questions: 48285 Slow queries: 9 Opens: 510 Flush tables: 1 Open tables: 92 Queries per second avg: 6.076
root@host [~]# mysqladmin processlist
+------+------+-----------+----+---------+------+-------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+------+------+-----------+----+---------+------+-------+------------------+
| 3485 | root | localhost | | Query | 0 | | show processlist |
+------+------+-----------+----+---------+------+-------+------------------+
root@host [~]#