Decommissioning an oil tank

[James Lusk]

I am putting an offer on a home in Lake Forest Park, WA built in 1916. The home used to run off oil but has since switched natural gas. The owner claims the oil tank was decommissioned but can't seem to come up with documentation. Two questions here. First, where could one check to see if it had indeed been decommissioned? Maybe city permitting? Second, if it has not been decommissioned, what problems could this possibly pose for a homeowner? This will be a personal residence in this case. Thanks!