Open the Registry Editor as an Administrator (log on as a admin or right lick the shortcut and click Run As)
In Regedit Click File > Connect to network Registry and choose a PC:
On the remote machine navigate to: HKLM\Software\Microsoft\NET Framework Setup\NDP
There will be a subkey for each version:
Check the Install value within the key to determine if it is installed.
The SP value will tell you what service pack is installed as well.
I.E. above you can see .Net v2, v3 and v3.5 is installed with Service pack one installed on v3.5.