Typically, if you need to discover the version of a piece of software, the convention is to go to Help | About.

Microsoft Forefront TMG is no exception, however TMG does not name the service pack or hotfix/rollup number in this information. TMG lists the full version and build number so to determine which Service Pack or rollup you have installed, consult this list of build numbers for Microsoft Threat Management Gateway:

Build Number Knowledge Base Article ID Comment
7.0.7734.100 n/a 2010 RTM
7.0.7734.151 977062 The IP address filter conditions of the filter do not work in Forefront TMG 2010
7.0.7734.152 977691 The application initialization fails after you build a Forefront TMG 2010 appliance if the computer does not have a valid IP address
7.0.7734.156 979578 2010 hotfix package: January 25, 2010 (not publically available)
7.0.7734.158 980311 2010 hotfix package: February 12, 2010
7.0.7734.165 980674 An IPsec VPN site-to-site tunnel or a PPTP VPN site-to-site tunnel does not work if you enable integrated NLB on a Forefront TMG 2010 array
7.0.7734.182 2520426 MS11-040: Vulnerability in Threat Management Gateway Firewall Client could cause remote code execution: June 14, 2011 (Client-side only)
7.0.7734.186 2616324 A hotfix rollup is available for Forefront Threat Management Gateway Client (Client-side only)
7.0.8108.200 981324 2010 Service Pack 1
7.0.9027.400 2288910 2010 Service Pack 1 Software Update 1
7.0.9027.410 2433623 2010 Service Pack 1 Software Update 1 Rollup 1
7.0.9027.425 2475183 2010 Service Pack 1 Software Update 1 Rollup 2
7.0.9027.441 2498770 2010 Service Pack 1 Software Update 1 Rollup 3
 7.0.9027.450  2517957 2010 Service Pack 1 Software Update 1 Rollup 4
 7.0.9193.500  2555840 Forefront Threat Management Gateway 2010 Service Pack 2      

Taken From http://social.technet.microsoft.com/wiki/contents/articles/1995.aspx.

Marc Grote and Karsten Henstrup also maintain a more comprehensive list of TMG, ISA and UAG versions.

It is also important to note that when a rollup or hot fix update is released, the information in Help | About is not necessarily updated. So the version being reported in the standard location may not be entirely accurate.

The most reliable way to discover the build number is to select the System node in the left hand side of the TMG Management Console and look at the version column.

Finding the correct TMG version number

Finding the correct TMG version number

Hat tip to Richard Hicks, Tom Shinder and Yuri Diogenes for pointing this out in episode 11 of From End to Edge and Beyond (mentioned at 29.19 in the video).

If you are also wondering whether TMG Standard or Enterprise edition is installed, this is clearly presented in the top right corner of the TMG management console.

Hope this helps!