The difference is really not being able to focus if it's not interesting. Your brain will constantly turn to other stimulation if it's not getting it from the current task. Sometimes you might be able to do it, and other times you won't. That's one of the most frustrating parts of ADHD. You feel like you've done it before, so why can't you do it now? "Normal" people can still focus (or get started) even if something is not interesting, they just have to start.
Most people won't like to do uninteresting things, but the threshold for boredom is extremely low in those with ADHD so they will struggle a lot more to stay focused. It's a spectrum unfortunately, so there's no one definitive measure that says "that's definitely ADHD". To be honest, if you find yourself seriously questioning/wondering about whether or not you have ADHD it probably means you're struggling to an extent and aren't 'normal'. Otherwise, it wouldn't be something you are questioning.