You can't really be sure of anything. There are some absolutely appalling games that are still being developed and some absolutely awesome ones that were abandoned.
With that being said I do notice some red flags like:
- The author starts to "experiment" with game mechanics, it usually means they have no idea what to do next and are stalling for time.
- Suddenly the game needs a remaster before being done.
- Now this might be justifiable if the dev started creating the game on some shitty laptop that couldn't do proper renders, but in most cases it just means the game will get milked for some time, then abandoned when the patrons jump ship.