The PreUpgradeCheck utility in SP2 is a simple to use tool that is run from the command line

stsadm –o preupgradecheck

This tool goes one step deeper and after the completion of this tool a nice output page is displayed that further dives into your FARM and outputs some decent information. If there are failures or issues it will be deeply highlighted in this output.

And the most important, it is read-only, so there is no hard to run on your server