To be a guide in Egypt the person must have completed, and passed, a three year course at university. They are then issued with an identity card from the Ministry of Tourism and they also have to apply for a tour guide syndicate card, both of which have to be updated annually. Anyone working as a tour guide without these cards is doing so illegally. So, if you have any doubts about the validity of a guide, when someone has approached you at a site saying they are a guide for example, just ask to see their credentials.
Be especially careful of people who say they are guides that may approach you once you have entered some of the tourist sites, or even the Egyptian Museum, and make sure you ask to see their credentials. Many of these are unlicensed and will try to scam you by saying things like you have to have a guide as you cannot go around the site without one: which is totally and utterly untrue.
There are some guides who will offer their services inside the garden of the Egyptian Museum. They are NOT employed by the Museum, even though they say they are, and care must be taken if you want to pay for their services. Ensure you get a low price, even teaming up with other visitors to share the cost.
If you do wish to get a guide, for any reason, ask at your hotel’s reception desk as they will often have a list of trustworthy people that they use.