Strength isn't a great indicator that someone will have visible abs, and while diet can play a role, there are people who have ab definition even when they're not dieting.
The truth is, abs can be a sign of multiple things. Here are six variables that affect their visibility:
1 – Genetics
The science of abs is pretty cool. Christian Thibaudeau has explained how some people – even when they get lean – won't be able to see their abs because their abdominal muscle bellies aren't naturally thick. (See: Abs Are Built in the Gym, Not the Kitchen.)
He's also explained tendinous attachments (they're what create the lines between abs) and how they can play a role in the amount of separation you see between abdominal muscles. In short, they determine whether you'll have a 4-pack, 6-pack, or 8-pack, no matter how lean and muscular you are. It's genetic.
2 – Training
Those who don't genetically have thicker ab muscles will need to do more work to hypertrophy theirs. Diet alone won't cut it. The abs are a muscle group, so this shouldn't come as a surprise. Train them directly with resistance. The big lifts alone won't cut it.
3 – Body Fat
There's a point when, even if you DO have developed abs, they won't be visible if you're carrying too much body fat. No amount of ab training will make them visible if they're underneath several inches of adipose tissue. You knew that, right?
4 – Digestion
Some people say chronic digestive distress can lead to increased body fat around the midsection. I'm not sure I buy that. But I will say that stomach distension and pressure in the gut (from gas, constipation, undigested food sitting in the stomach, etc.) can make your belly protrude. And extreme protrusion alone will make ab separation less visible, unless you flex hard.
So even if you're someone who's lean enough to have abs, slow motility and poor digestion can simply make it harder to display them.
5 – Water Retention
Ask any fit woman when she wants to get professional photos taken and there's a good chance she'll schedule it around her period. Why? Because there are a handful of days every month when we retain more water than usual, and it can affect the appearance of our midsection.
But even men experience water retention for various reasons. Sometimes both males and females will use diuretics before big events in order to manipulate water and increase definition all over, including the abs.
6 – Skin Tone
Ab definition is slightly more visible when you have a tan. It just makes what you have easier to see. This is especially true if you're lean enough to have ab definition, but you haven't been able to build much muscle thickness there. So if you're pale skinned and you have a shadow of an ab or two, try slapping some fake tanner on and see if that makes them a bit more visible.
To recap, having visible abs mainly depends on what your mom and dad gave you, how much muscle you've built in the midsection, and how lean you are. Contributing factors may include digestion, water retention, and skin tone.