Is it possible to judge a possible spectator whether they will be more intrigued by coin magic or card magic? Or is it simply a judgement that is made during the performance through trial and error?
I'd agree with this, I think it's more important to be able to tell if they'd like to see you perform or not. You shouldn't ever go up to a group and immediately start performing, first introduce yourself, ask how they are doing, get to know them. Then explain you are a performer and ask whether they would like to see you perform. You should also remember that you have the right to deny service if they seem like they would be a disruptive audience.I've never really tried to gauge if they'd like a certain type of magic over another, but more so if they'd like/be receptive to me.