Verifying an Sxx, Cxx, or Oxx patch update was applied and is being used in MailStream Plus

MailStream Plus™, all versions, all platforms
This applies in general to all patches and updates that might be applied on top of existing versions of the software.  

When you run a MailStream Plus™ (MSP) job, it produces the XLG output file.  This file will normally have a .xlg extension on Windows or UNIX/Linux, it's the DD MSRPXLG on z/OS, and the PRNTXLG on IBMi. 

The XLG lists every time a program starts, and at the right-end of that line, it gives the version number of that specific program.
For example-
03/10/2017  17:02:16            MSXXVP00   I MSXXVP00 started. Comp 12162016 at 155118. Gen 12162016 at 155018. R08.3M03 F03.
That means it's MSP version 8.3.3, the F03 build.  The F stands for Full, so it's from the Full version build of 8.3.3 that was released to users.  In a Full release, all the programs have the same 'F' code.

When a program is later updated (or 'patched') by a Sxx, Cxx, Oxx, it will have a different identifier there. 

As an example using the the 8.3.3 S01 update- 
Look in the XLG output file of a job you ran after applying the update.  
Look for the identifier.  For example for the 8.3.3 S01 update, search for 'S01'.  You will find a line(s) like this, with S01 at the right-
01/31/2017  15:23:11            MSPSWD00   I MSPSWD00 started. Comp 01202017 at 154853. Gen 01202017 at 154824. R08.3M03 S01.
So the MSPSWD00 program was updated when S01 was applied.

Or you can search for one of the programs that were updated in S01.  In the 8.3.3 S01 update, MSNA00 was also updated (there may be a letter to the right of MSNA00 like A, B, C, or D.) Search for 'MSNA00'.  
01/31/2017  15:23:18            MSNA00A    I MSNA00A started. Comp 01202017 at 154850. Gen 01202017 at 154833. R08.3M03 S01.
In some cases though, specific updated modules may not be called and used in specific mailing jobs. But using method 1 above will generally find a program that indicates it's from the new patch version.

'F' stands for 'Full release'.
'S' stands for 'Single'.  An update that generally (but not always) was to correct a single issue.
'C' stands for 'Combined', an update that generally fixes multiple issues, and may include previous Single updates that were released.
'O' is for 'One-off', an update that is made and provided to just one customer (or a very small number of customers) to address one specific issue, but the issue does not affect the majority of MailStream Plus users.
UPDATED:  July 1, 2019