how do you know for sure that your a bronie?

When you watch and like the show and you are an active part in the community.

It's not really whether or not you know you're a brony. It's whether or not you believe you're a brony.

You can like the show without being a brony. You don't have to call yourself a brony simply just because you like the show or interact with the brony community.

Look up Joshscorcher (A.K.A. The fiery Joker) on youtube. He has done MLP episode reviews, and he has actually said he likes some episodes (for instance, he said he "loved" the royal wedding episode). He has even interacted with the community in some ways, like going to Bronycon.
However, he has actively stated time and time again that he doesn't consider himself a Brony.

So to answer your question: I believe you just have to like the show and actively consider yourself a brony. There's no "knowing" involved. You simply have to "believe" you are one.

Essentially what btflash said. Being a brony is a choice, not a status/rank. If you don't consider yourself a brony, then you aren't. The donning of an internet surname isn't mandatory to do anything that they do.

Brony is a pretty basic thing: It says "I like My Little Pony Friendship is Magic" in one word. The connotation of that could be "MLP fanatic", but nonetheless, it's still just an optional title. People shouldn't look more into it than that. It's built up a stigma and brings about idiotic generalizations about the fandom, so some fans choose not to use the word "brony" to describe themselves.

The gist is that if you think you're a brony, cool, if not, then it's the same matter; you can't be treated differently just because you choose not to use the surname. It's all the same to me.

When you doze off frequently thinking of ponies. :D

When you're obsessed with MLP:FIM.

I'm not a Brony though.... :/

I'M A PEGASISTER!!! :D

You have watched too many MLP FiM.

Like the others said being a brony or a pegasister is a self dubbed title but I believe that there are levels of broniness
(each lvl includes the previous ones)
Lvl 1 u consider yourself a brony and u watch mlp fim
Lvl 2 u r an active community member and know all the mane sixes names, element and traits
Lvl 3 u know about the cmc, zecora, spike, silver spoon, diamond tiara, big Mac, cherrilee, granny smith and some jokes like 20% and draw only ponies
Lvl 4 u know the famous background ponies lIke derpy, Bon Bon, Lyra, and Octavia and can quote some of your fav episodes
Lvl5 u know all the background ponies and can recite the plot lines of your fav episodes, quote all the episodes, know some songs by heart
Lvl 6 u know the plot lines of all the episodes and know most songs by heart
Lvl 7 u know all the songs and the famous fanfics like cupcakes and write your own