Look at his mannerisms. He's very relaxed, nonchalant and so on. Not until he breaks his neck do we see any real force from Cell and even then, he doesn't appear to be going at it all that hard. He's a bit harder to judge because you can't read his facial expressions, but his overall demeanor is so relaxed, it can't help but be noticed if you look for it. There is no strain on Cell. He's just casually going after Piccolo and Cell like it's a walk in the park.
After Piccolo is taken out, Cell asks 17 if he's tired of playing this game too. It's clear that Cell has upped the ante here. He's leaning into his punches with his body more, the art indicates effort is being used and lookie here, Mr. Infinite Energy is put down with a 2 shot combo, so much for never fatiguing and not feeling pain.
Now look at Cell's posture after being attacked by 16. He's much more aggressive now, the look on his face, much more serious. He's in an athletic position. The art indicates Cell moving faster, he's using his real ability now.
If you are unable to tell when a character is going for a casual stroll or when he's actually giving 100% effort, I don't know what to tell you. It's all right there in the story. The author uses facial expressions, swirls around the characters, noises around the characters, posture and other tells to denote what the story is trying to express to you. Why don't you try using your eyes next time? Don't just look at every power statement and think that's everything.
If you go by power statements alone, we would never know Vegeta is weaker than Initial Final Form Freeza. You are already living here many hours a day for years, you still haven't picked up on this shit yet?
