I test numbers.
When I don't hear an Indian pickup but an EU/US/CAN/MX person instead, my spidey senses tingle.
When I ask pointed questions about where XYZ's office is located and get back a clear, quick answer, they tingle more
When I ask for the mailing address of the firm and it's read back out to me instead of an angry hangup, they tingle a lot
And when I go to the firm's legit website and see the legit number listed there, alarm klaxons are going off and then I get mad.
I get it that spoofing a real firm is in the scammers best interests and they do that, but as an example one account that I reported kept posting Portfolio Recovery's DID numbers in here. I know they're real because you can go to any number of consumer law firms who specialize in suing debt collectors and they SEO the numbers so debtors can find them in search rankings.
And I know Portfolio's IVR by heart---they've gone after me for zombie debt that's out of statute. Another number posted here was of a local FBI field office, and I know that IVR as well and that number is as plain as day on the FBI field office's homepage.