The game has more middleware logos than actual permanent programmers.
It is developed by some studio you've never heard of, or even moreso, the developers are massively downplayed.
Sometimes like in boxing, a developer is a jobber. It's their job to be bad. Take Arzest, Game Loft, and Data Design Interactive.
Rather than standing on the merits of the actual product, the marketing refers to it as an X-Killer or directly dunks on some other game.
This is subjective; but if you look up the dev team and find either: An alarmingly high ratio of dyed to undyed hair or a fursuit, one may wish consider this a red flag.
An easy one is refusal of interviews, sit downs, or conferences.
Whenever an old brand logo appears, and yet none of the old talent is there. Say anyone invoking the name of Acclaim, Atari, or so on. Moreso when the studio was a questionable name in the first place.
If you need a spreadsheet to figure out what the preorder bonuses/special editions are, that's a sign that there was more effort put into by marketing than the developers.
When you get the feeling that hypothetically you could run the game on only 256 colors and most of them are the same exact tints and shades of gray/brown/murk.
During the previews, they show the map ingame, and you can't actually see any of the map contents due to a plethora of tags.
The website for the game either doesn't exist, is grossly overfilled with marketing buzzwords, or is something absurdly vague, like a countdown. If you're proud of a product and want to pump people up, you'll show it live, warts and all.
Attempting to puff up map size as an indication of anything. Minecraft smashed the vase years ago, let it rest.
A delayed game can be fixed. A rushed game will have a forever tainted release, even if it eventually gets a positive turnaround like NMS.
When the tutorial is explicitly referenced as such, instead of seamlessly being integrated into the game or put to one side as an action guide. (This is recent.)
When you find the materials of the game using made up words to reference real concepts; forcing you to read an ingame glossary.
If you have to refer to external materials in order to understand the whole plot or miss on critical things because you forgot that Mr. Berkshire on page 122 of the extended universe manual mentions the Excalibur 2 was dropped in the sewers.