Sorry, are you suggesting that able-bodied people never take a seat, or that we always ask if someone would like to sit even if they don't look ill? I hope you realise why neither of those are practical. It is reasonable that an able-bodied person with a seat should ask if someone who looks like they need it if they want to sit instead; but if your illness is invisible, it's also reasonable to expect that if you need a seat you'll ask. It's not reasonable to expect me to know you have an illness and to offer my seat if you're not outwardly ill, though.

I suspect you may be thinking of people refusing to give up their seat when you ask, because you don't look ill. Those people are jerks and that reaction is bad, but they are not what I'm talking about here. We're discussing the best way to ask if someone needs a seat, without offending by assuming the reason they might need one.