Thanks for this quick description. I did try this will both the email and the message posted by dexX7 above (twice each) and I get a message "verification failed: no signature found." That seems strange to me because I can see the signature clearly at the end of the message.
Not sure what I'm doing wrong?
Egad, right you are! There's a limitation created by the file type, which I didn't pick up on. The ".textClipping" file type supports formatted text, which means it does not contain just the text you selected in your browser, but also extra information for formatting, such as the boldfacing. GPG will cough on that.
If you instead save the text to be checked to an ordinary text file (e.g., paste into Text Edit, set as plain text, and save) rather than trying to validate directly on the ".textClipping" file, then at last you'll be good to go.
Ok after a few more tries I did verify the file. Thanks for your help DrGregMulhauser. I need to watch a few more videos and read a few more webpages before I have the hang of this.