Around the midpoint of a game, often while still enjoying it, I start asking myself how close to the end I am. Let’s Plays are great for this. I can just open a playlist and read episode titles until I’m around the same point I’m actually at. Let’s say e.g. 45/67 videos or 2/3. If I rarely do this, maybe even not at all, I’m really into the game.
For context, I mainly play quite linear JRPGs. For other games, I usually just look at howlongtobeat.com and compare it to my playtime.