Most physicians, nurses, or therapists have to take abnormal psychology as a pre-req for a reputable university diploma. Things like Munch used to only garner about a twenty minute lecture, if that, and then you moved on. However, there was a bit of a tidal change in the 90's and you started to see some larger focus on the topics of malingering and factitious disorders. Now, you can take entire classes that revolve around the physiology and psychology of MSBP (Munch by Proxy), Munchausen's, and other disorders that clog the system.
I'm hoping it's ok to power level a little since it was a question that was asked... I'm middle aged and when I went through school, we didn't get any education, beyond just knowing what the definitions were and your standard child abuse talk. But, I've taken entire post-grad and secondary CMEs on spotting and confirming Munch. What you're looking for depends on two very important factors: who is claiming to be ill and what are they presenting.
First, if it's a mother claiming that her child is ill or on death's door, but she's completely calm, you know you're dealing with a potential munch. Most parents over react- that's instinctual- because we want our offspring to be healthy and when they're not, it's scary. But, when you draw back the curtain and mom is texting or insists on giving you more than a ten word answer to the question: "What seems to be the problem this evening?" - your radar goes off. 90% of the time, if you say "What's wrong?", a normal parental response is "Her temp is 104 and she's not acting right." A non-normal response, in an emergency setting, is "Well...so when she was 6 months, she had x,y,z . We saw Dr. Who, and he referred us to *insert name of fancy university* and they did tests and blah, blah, blah." All that tells me is that you're not overly concerned about your child ACTUALLY being ill because you didn't tell me what's wrong. The other red flag is hospital jumping and doctor shopping. If you've had 10 abdominal surgeries at 8 hospitals,...I'm concerned.
If it's by proxy (whereby a parent or guardian is claiming shit that just doesn't add up) , then our hospital policy is actually a direct admit for observation...and we have things we do that test the parent (but, I'm not sharing what those things are because I don't want to give a hands up to a munch mom/dad). Usually, with the admit, you'll see a weird glee in the parent and they'll call EVERYONE to say how sick the kid is and start prayers...and I guarantee you it happens more than you know. Especially in this day and age. I had a mom who didn't know that the glass on the cubicle was giving us a perfect view or her tablet screen and she was typing up a GoFundMe page for the child we were evaluating...when she was told we were admitting for observation, she posted the GoFundMe. Yeah. That bad. Another obvious tell is when the kid starts tanking when that parent is present. MBP perps are narcisists at the core and they usually fuck themselves by not shutting up.
For spoonies, there are tells, too. If the patient has a history of visits for things that were serious enough to come to the ER, but magically resolved before the doctor or nurse came in? "I swear I was covered in hives five minutes ago." Or, "I can't breathe!" Well, you're talking to me, so I say you can breathe. "I hurt everywhere! It's a 10/10....ow...can you hand me the remote?" Or, my personal favorite... you'll hear them talking or laughing from the nurses station, but the minute you hit the door, they remember to double over and moan. Fun trick- loudly declare that you're going to go check on Bed 3's pain. The lols are numerous. They will amp up that moan to 11. Uh...what else? They have a two page list of various shit they've been diagnosed with, but have no physical findings that match. They have 10 meds they claim to be on but don't beg you for the stuff that doesn't give them a buzz. They argue and throw a fit when you clear them to go home. OMG!!
I've never done this....but, a guy I work with has a little trick he likes to call "Positive Apple Sign". While talking to the patient, who is usually holding a phone, he'll drop a blanket or towel on the ground and lower the bed. He does his exam and woe the patient who claims 10/10 pain or "It hurts so much I can't move" but still talks normally and doesn't show any guarding. He'll pick up their phone and accidentally drop it and almost ALWAYS , if they're faking , they'll go for that phone like it's the diamond on Titanic. A less harsh version of that is to move the phone to a counter for the exam and then leave it there....you'd be surprised how someone who can't walk gets their shit back like they have a Hogwartz wand.
fftopic:because it's an IRL rather than internet, but I do think it's relevant. 
