Yes, adult fleas are big enough to see with the naked eye, although it can be helpful to use a magnifying glass to see them.
They are either black or reddish-brown and look like tiny dots and can be difficult to see, although they are visible. They have a hard and callous shell which makes it difficult to smash and kill them.
Fleas in other stages of the lifecycle such as eggs and pupa can be more difficult to see with the naked eye. They are usually white colored and can be found mixed in with the bedding of the animal that is infested with fleas.
There will usually not be only one flea, but many on a host. Fleas reproduce quite rapidly and when they successfully find a host, their numbers increase rapidly in a short amount of time.