If you're particularly interested in this house, and USTs (Underground Storage Tank) are a concern of yours, this particular house has gas fuel and steam heat.
For good or for ill, the fuel source is not listed under the features (as you know) and thus you'd have to either contact the broker [(good marketing -- less (information) is more (responses)] or, if you're working with a broker, have him or her find out for you.
With that being said, however, there's no guarantee from that information that there was never a UST on the premises. There also might have been an above ground oil tank in the basement. One could ask to see the Seller Property Disclosure and/or if you move forward have your attorney include a rider in the contract that requires the seller to disclose and/or represent (legal term) that there has never been a UST on the premises.
If you want to pursue it even further on your own due diligence, you can check with the local buidling department to see if permits were ever issued to properly decommission an oil tank on those grounds. You could also check with the local utility company to see how long gas has been the fuel source in the house. If there's a discrepancy between the year the house was built and the installation of gas, it might be a clue that oil might have been a fuel source. On the other hand, depending upon how old the house is, the fuel source might have been coal.
If you're represented by a good attorney, I'm sure that he/she will have experience in these matters to protect your interests.