Probably not, as it would make any duty-to-care legislation toothless, as the excuse could be tried in all circumstances: "Your Honor, after I drove into the pedestrian, he was under my car, but I was sure he got there voluntarily, so It couldn't have been that bad".
Since A is not a medic, he's unable to make up an appropriate diagnosis of B's state. That's not expected of him, but common sense would expect him to know the real signs of e.g. respiratory distress. Yes, that probably means touching the patient, but nobody is able to diagnose somebody without looking and touching. And "Well, he looked like he's simulating" or worse still "Well, he looked like an impostor" is unlikely to convince the judge.
Normally, you can't do much wrong by calling an ambulance. Even though there have been disputes about who had to pay for the ambulance when the victim didn't actually need nor want it, this is likely the smaller problem than the sentence you'll face for not helping. Also, this doesn't apply here, as B is arguably unable to say anything.
Note: While googling, I found this report, which states that particularly in China, calling for help is a common fraud scheme. Particularly elderly people simulate an accident, and when someone helps them (and brings them to a hospital) the helper is accused of an assault.