Sorry to hear that you’re uncertain if your acquaintance has been knighted or not. It’s quite improper to use the title “Sir” or “Dame” before your name if you have not been honored in an honors system. The best way to find out is to search for records of his knighthood - or her damehood, if female. I have included the easy way to search for British knighthoods in this post, since that is the system I know best. Other honors systems will require different methods.
First, make sure you have his or her proper name. If presented with a card, it’s a good guess that the person is using the name under which he or she was knighted.
Second, access The London Gazette online. It should be the first hit in a google search for “The London Gazette.” The Gazette contains official British public records.
Third, (as of the writing of this post) type his or her name into the search bar to the middle right of the screen. Press the magnifying glass to the right of the search field.
Fourth, read through the listed postings. There might be a few, but a knighthood will often be on the first page. Chances are that the screen will show some records such as “The Queen was this day pleased to confer the honour of Knighthood on …” in the block caption. Obviously, finding the entry means your “friend” was knighted. Search EVERY entry until you find it. If the record is not there at all, double check that you spelled the name correctly. If the search STILL yields no record, then you’ve got an imposter.
Fifth, congratulate your acquaintance on receiving a knighthood or damehood. Make sure to ask if the Queen beknighted him or her at Windsor herself… or if someone of the Royal Family was there instead. This works whether you found the record or not.
Finally, if all else fails, you may contact the Chancery and kindly request records on the knighthood of “Sir…” Remember that one may say “Sir Wyville Thomson [Sir First Last]” or “Sir Wyville [Sir First],” but never “Sir Thomson [Sir Last].” The address of the Central Chancery is:
Central Chancery of the Orders of Knighthood,
St James’s Palace,
London SW1A 1BH
Now I will leave you to wonder how often I search for records of knighthood. I hope this helps.