You look for unusual patterns, hooks in the pattern, clear bellies, or bellies similar to other subtle morphs. Look for uber light babies, or uber dark babies. Just something that looks different enough from wide range of normals that you think it's worth trying to see if you can prove it out. If you can't, well, you still have a cool animal in your collection!
Here's a dinker girl of mine:
You can see a hook off of her back pattern:
Clear belly
It does take time just perusing sites and seeing what others see in their dinkers, especially those that prove out. Eventually, you'll get an eye for what's different.