This is an unfortunate generalization, if only that I agree in a very large part and do it anyway for lulz.
You know a map is bad when the post is 4 chapters long & the text is photo shopped into the thread with crazy analysis of sight lines & power positions & then you retire...
You know a map is bad when the one screenshot of the map is in the first person POV. You know a map is bad when it uses more then 6 toys at once.
When the screenshots look interesting but then within a minute of running around on the map the disappointment sets in. Or when the map feels more like a campaign playspace than one intended for multiplayer.
You know it's bad when a forger who constantly gets raped and spends more time on forums and in forge lobbies critiquing maps than actually playing at competitive level... likes it.
You know a map is bad when you ask the author what their inspiration was and they say "well i'm a big fan of Lockout, and..."
You know a map is bad when the author spends the first 2 minutes of the walkthrough video standing on the initial spawn. You know a map is bad when you boot it up in a custom game and there are no intro cameras
You know a map is bad when you look through 10 screenshots and a video and still don't know what the layout is.