I have an oil painting of highland cattle by Louis B. Hurt dated 1913 and titled "On Rannoch Moor". Size is appx. 22-1/2"x17-1/2". How can I find out its value?

You may wish to look into Art prices, art appraisal - Search free. They have many listings for Hurt, with prices at auctions, without subscription. If you wish to see pictures, subscription is necessary.

I also like to Google in the artists name plus "gallery" or "auction." This
Often brings up good things.

Try Christie's and Sotheby's. Look up auction archive information.

AskArt is a nice site, but subscription is required for thorough info.