Poking around the registry, it looks like
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSBuild\ToolsVersions\2.0
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSBuild\ToolsVersions\3.5
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSBuild\ToolsVersions\4.0
may be what you’re after; fire up regedit.exe and have a look.
Query via command line (per Nikolay Botev)
reg.exe query "HKLM\SOFTWARE\Microsoft\MSBuild\ToolsVersions\4.0" /v MSBuildToolsPath
Query via PowerShell (per MovGP0)
dir HKLM:\SOFTWARE\Microsoft\MSBuild\ToolsVersions\