Find MySQL Patch Level
There is no command to execute for this. What you need are the following:


Oracle MySQL Executive Summary (under Supported Versions Affected)
MySQL 5.1 Release Notes


I would be concerned because MySQL 5.1 was last updated Dec 03, 2013 and the Last Critical Patch to MySQL 5.1 was Jan 2014 (See Patch Availability Table under column Patch Availability and Installation Information) 

On Oct 17, 2014, I posted the answer to Any known issues upgrading from MySQL 5.1.73 to 5.6.21? following the security patch announcement. MySQL 5.1.73 has had no security patch applied. Only MySQL 5.6 (5.6.21) and 5.5 (5.5.40) have new patches.

What's crazy is that MySQL 5.6.20 was released two months prior to MySQL 5.6.21. Again, no mention of MySQL 5.1.73.

I highly recommend you upgrade to MySQL 5.6.21.