You go to your nearest auction house that sells pictures, inter alia. They will have books of artists which tell them a) who F T Underhill was b)what records there are of this artist's work being sold and at what prices. They usually have someone who can judge what it will fetch, even without that information, judging it by the style , period, and craftmanship, because they know the market for such works.
Where are you? I can probably identify your best local house.