As I use and understand the terms, it's about the difference between the physical and mental elements. If you love to be whipped, spanked, tied up, whatever, but still remain your own person, accepting it as something done by the top for your pleasure, you're a bottom. If what excites you is to feel controlled and dominated - whether that control is expressed by beatings, bondage, or just kneeling at your Dom(me)'s feet - you're a sub.
The acid test is, how do you respond to something you don't enjoy, but your partner does? If something that never attracted you before, suddenly becomes exciting because the right person is ordering you to do it or forcing it on you, you're a sub.
The first time I was married to a switch, we could only make it as Master and slave because when she switched she was a top, with no particular interest in dominating or controling someone, just hurting them big time; and when I switch I'm a sub, pain alone does nothing for me, but pain inflicted as part of controling and dominating me is a rush.