I do not have Autism or Aspergers, at least I haven't been tested for either, but I have very very similar issues when it comes to socializing. I keep feeling like I am missing the gene that makes socializing so intuitive for others.
One thing that helped me when it comes to recognizing the difference between flirtation vs friendliness is that people who flirt will push the boundaries every now and then, while people who are just friendly would stop their 'flirting' behiavior once it reaches a certain limit, like achieving their goal of being humorous. People who flirt to make you and themselves feel good stop once they reach that level; those who are interested will keep going, they will keep talking, keep seeking you out, and generally just try to find excuses to continue their flirting.
Now the above only works for those who are confident enough to be foward in their desires, usually extroverts and social butterflies. To find indications of interest in the less outgoing crowd is a bit more difficult. This is when reading body languages can help, though that in and of itself needs practice and is hard to decipher. I know it is difficult for me sometimes to navigate a conversation and focus on a person's body language at the same time. I usually sacrifice one for the other in a social situation (imagine some of the awkward moments I found myself in when I had no idea what the guy was talking about because I was too intent on trying to see if he is doing a crotch display or wondering if I should stare at his lips or his eyes while he is talking!)
However there are a few things that can help in terms of body language that is universally true. If you look at a girl and she catches your eye but looks away, keep your focus on her- if she looks back after looking away then she is interested, at least initially. This is especially true if she smiles or blushes or looks down when she notices you looking at her.