Your guide should have a physical badge, like the one pictured. They will probably be wearing it, either pinned to a lapel or on a lanyard, when they meet you. If you can’t see it, you are entitled to ask them to show it to you. You will also find them listed in the Institute of Tourist Guiding Directory – here’s my entry.
Guides should also have an ID card confirming the badge they hold and their membership of a professional organisation.