As other answers point out, there are some requirements on what must be contained in the notification, but nothing about any particular authentication mechanism.
So, the upshot is that, if the notice meets those requirements, and if it did in fact come from the real landlord, then it's effective, and you will be responsible for paying the increased rent without having to be asked again. That is the case even if the notice isn't signed, or isn't on letterhead, or doesn't contain any other particular proof of authenticity. Therefore, you have to judge for yourself whether it is authentic, or whether to take additional steps to verify the document. If you end up guessing that it is fake when it is actually real, you will bear the consequences.
It's up to you to make your own cost-benefit analysis, given the circumstances, of the effort involved in verifying the notice, versus the consequences of being wrong. You might for instance decide:
"This notice appears so obviously fake that I'm just going to ignore it. There may be a tiny chance that it's actually real, in which case there could be trouble when I don't pay the higher rent. But that chance is so small that it's not worth the effort to contact the landlord and confirm that they didn't send the notice."
"This notice appears so obviously authentic that I'm just going to assume that it is, and plan to pay the higher rent when it comes due (or, plan to move out before it would take effect). If it turns out to have been fake, I will have been inconvenienced by paying too much and having to wait for the landlord to refund the balance (or, by moving out unnecessarily). But that chance is so small that it's not worth the effort to contact the landlord and verify that the notice is real."
"I am not really sure one way or the other if this notice is real. If I guess either way, the risk of being wrong would be substantial, and the consequences would be unpleasant. So I'm going to contact the landlord to verify it before doing anything else."
There are lots of things in the law, as in life, that are like this. You're not guaranteed to be given absolute proof of whether something is true; you have to make up your own mind, and decide how much effort it is worth to make sure you are right.