Specific red flag signals:
1. Dominated by Windows
2. Too many people in formals
3. Nobody wears badges (and other indicators of a lax security policy)
4. Old/worn out office furniture
5. Low resolution displays
6. Old laptops
7. No evidence of people collaborating or chatting at a microkitchen etc.
Depending on the size and age of the company, most of those hint towards a product-focused organization that often cultures careful, reliable engineers. It might not be fun or flashy, but there's good odds that a place like that knowns how to build their product and does a consistent job of it. They're often a great place for senior engineers to settle down and focus on low-drama productivity and for junior engineers to witness durable best practices in action.
Salesmen have Mac Pros or fully decked-out gaming rigs, programmers are still on Pentiums.
But seriously, we'd fail 1, 3 (we've never had anything like badges to wear), 4 (some of it is old, not too much us worn out though - chairs get replaced, desks are pushing 25 or more now), 5 (every one, including receptionist has multiple monitors but they're standard 300dpi), 6 (depends what you mean by old), probably 7 as well as most folk are remote now.
I guess we're not for you.
On the up side we dress very informally.