Visual signs of distress and vital signs are a really good start. People in real pain don't hide it well for the most part. Grimacing, grunting, labored breathing, perspiration, elevated bp, hr and so on are all good indicators of real pain.
Calm, minimal or no distress, flat vitals and directly asking for pain management for a vague, chronic pain complaint are all indicators of a bs call. Not absolutely bs of course. I took a guy yesterday who slipped on the ice and broke his ankle. His foot flopped around like a wet lasagna noodle. Multiple fractures confirmed on X-ray later at the hospital. He was the chillest dude ever. A bit hypertensive but nothing crazy. All other signs unremarkable. So there are people out there who are undercover sick.
All that being said, when I wrecked my motorcycle and wound up in an ambulance, i was grossly undermedicated. It was ultimatley hours of ineffective small doses of pain medicine before one of the trauma surgeons at the hospital decided to nut up and get to it. Consequently, having lived through the side of the patient who's pain is being under treated or worse, ignored, i will give drugs to almost anybody. I truly believe a patient experiencing real pain deserves relief from that pain regardless of whether or not their injury or pain impresses me.
The only people I dont medicate are the obvious drug seekers. Beyond them, if I believe your pain is real and you want relief, I'm on it. I too have been burned. We cant put peoples pictures on the station house wall labeled "med seeker", so inevitably we're each going to encounter these people without knowing their game. I dont care. I'd rather unwittingly give a hundred junkies a free pop than let one patient with real pain slip past me. This causes me some grief when I'm working with a VOMIT medic, but I'm past caring about what lazy hacks think about my practice.