If the target succeeds at their wisdom throw and the spell falls, there are two options that most DMs will use. The first is that if Charm Person falls, then the spell never got a good grip on the target’s mind and it’s just a wasted action and a spell slot. Other DMs rule that the reason the spell failed was that the target was actively resisting the spell, and they would know that they were being charmed. For example, let's say you tried to charm an orc. The spell fails and the orc fought it off, and per the rules: When the spell ends, the creature knows it was charmed by you. Then the orc would know that you tried to charm it because technically, the spell ended when he fought it off. It’s a little confusing and most DMs will handle the rulings on the spell differently whenever it comes up. There will be a house rule for it, once it is established, you might be a bit wary when it comes to using the spell depending on what side of things your DM lands on.