Ask HN: Strategies For Detecting Scams

So I recently received a call from a debt collection agency. They exhibited all the signs of a scam, and when I looked them up there were tonnes of reports of them being a scam. On the off-chance it was real, I called them back and requested the information on the charge, which although it seemed to be related to a legitimate doctor's visit, was from a diagnostics company and debt collection agency that the doctor's office denied all knowledge of. I called the company that the doctor said they sent the test to, but they had no record of me. I threatened to contact the FBI about the calls (from the collector) until finally they gave me the complete information (which they all knew all along). That was also when I started getting polite operators that were cheerful and helpful. The doctor's office sent a test to a diagnostics company, who forwarded it to another company, who forwarded it to another company, who employs a doctor who did the test, who charged it to my insurance company. The insurance company denied the charge, and that's where it ended. They all claim that someone else should have billed me, but when the company that actually performed the test didn't get paid, they sent me (not the people they claim to be responsible for billing me) to collection. That was an hour of phone calls, and now the one company I need to speak to in order to resolve the issue is closed for the day. Since I was so sure the collection agency was a scam, does anyone have any advice on how to be sure something is a scam, when you think it is?