Several possible ways.
If it was fairly recently, check the age of the gas furnace.
If permits were required, check your local city permits office to see when permits were pulled for the conversion.
Call up the gas company and ask when that address first was serviced by them.
However, from your additional comments, your question (and my answers) won't get you to where you need to be. Seems as if you're interested in whether there was oil leakage. That's an entirely different question. If that's the issue, a home inspector should be able to help determine that.
Hope that helps.