One of the most interesting things about Laughter, is that it is a social response. Typically, people will tend not to laugh at things while they are by themselves, even if they find it funny. You are more likely to respond positively to a joke, by laughing, when surrounded by other people.
We laugh to show other people that we are amused, having fun and enjoying ourselves. Indeed, we do it to make a connection with other people. Thus, one of my rules for telling if I find something truly amusing, is if I laugh at it when by myself.
I always feel embarrassed when I am by myself, and listening to a podcast, for example, on my commute, and people see me laughing at apparently nothing. If I see other people laugh to themselves in the street, I usually assume they are listening to a podcast. At least, I usually hope they are.