Mmmmm, good question. I imagine myself walking in the street and someone calling me by my name, or me at work and receiving a call from security advising that someone is requesting to see me. I see a normal guy or woman, no special phisical carachteristics (since I believe that mainly all ET's are the same as us. Same plot, in other words).
He/She would ask me if I knew who he/she is and I would know.
Perhaps a certain look in his/her eyes, some kind of strange way of talking, or I simply would know as I simply know many other things.
If I got any possibility to choose, I would like he/she to be an extremely intelligent and kind person who could answer all my questions and teach me everything I desired to know (and that is a lot of things). He/she, should be a person with an excellent sense of humor. And by now, I don't have any more ideas about him/her so we are going to stop it here or I will be banned.
I just can't wait for that moment!