Are the guests in Westworld forbidden to tell the hosts that they are robots?

No; just like in Season 1 (where Dolores finds a photograph of a woman on a busy NY street), the hosts are programmed to suppress and ignore any indication that a world outside of Westworld exists.

Bernard explicitly shows Dolores the photograph of the woman, and asks her what she thinks about it. Her reply?

"Not much at all."

There is a scene in which William is being helped by a host to prepare for his "adventure" and she offers to be intimate with him, to which he asks, "Are you real?" and her reply is, "If you can't tell, does it matter?", indicating that the (or at least some) hosts are already aware of their condition. For the ones that are not, John Smith Optional's answer applies.

My point is that the hosts cannot understand what does mean to be a robot. They are just beings, and humans are beings, too, but not only that, even the similarities between the hosts and guests add to this lack of perception of any difference between them and the humans. There is no presumable identity between them and humans, either, because even the hosts are not all identical between them. So there is a generally existing small variation between all beings (among any each two individuals from the Westworld scene).

Add to that the programming issue (in which each host is meant to follow an own narrative line) so that there are very few reflective guys among hosts, aka robots contemplating and inquiring the very nature of their own reality.

Imagine that you say to a dog that he/it is a dog and not a human. Who cares? Not him/it, as he/it is moved only by his/its desire to eat, bark, howl or sleep, and nothing else.

Anything else is not important at all for its nature. As the definition of ignorance says (the ignorant thinks that everything that he doesn't know it's also not essential - so being ignorant is a choice, not a given), the same way hosts are programmed to not care (to avoid) what's outside their own way of thinking, and also to return always to their original narrative line.

I would also add to the selected best answer of this question the fact that when asked "what she thinks" Dolores analysed what she felt about that think (which was nothing, of course). If she would feel anything about something, that would create commitment (as it was the commitment and dependence for her "father"). Obviously, when you play a role, no feelings or commitments are possible for anyone.

Remember that Dolores would smile to anyone that would pick the metal can that escapes from her hand. To anyone, without any feeling for them. Therefore, in the beginning, she cannot understand too well the nature of the guests and also of her reality. She didn't care, actually, as she did live like in some plastered happiness ...