Won't help for phones, but I did learn a little "face reading trick" a while ago.
When people are making a joke, and they don't laugh at it themselves, they'll often leave their mouth slightly open like a teenager. As if waiting to keep on talking, but needing a breath first. I think this is because they are waiting for the other person to laugh to give them permission to have a little giggle at their own wit.
So if you find yourself in a face to face situation and you are wondering to yourself if what was said was a joke or serious, look to the mouth. If it is slightly open then there is a good possibility that they though what they just said was funny. Of course, there are other reasons people have their mouth open, so the last little give away is if the mouth twitches a little bit.
Not fool proof. And won't help with the phone, because there is nothing to look at of course.
As for the topic, I have no idea how you want to tell her. For my dad I just told him straight up, my psychiatrist thinks I might have autism so we are looking into it. I told him when I was still looking into it, so his knowledge sort of gained with mine. With my mother it was more of the same. However I have a history of being diagnosed with different mental illnesses (one's that never really fit until Autism came along.) So I've had lots of practice with saying... well, this is what the psychiatrist thinks it might be now... type deals.
Can't give advice for someone looking to explain it to someone who already has negative connotations associated with that diagnosis, and mental illness in general. Hope you work something out though.