If they are disabled in such a way as to require a seat, they are probably using some kind of walking aid. A blind person will generally have a white stick and/or a guide dog.
It's arguable whether "a bad back" or "a sore knee" is sufficient reason to qualify as disabled, unless medically certified. The term "disabled" has become very loose nowadays. If someone has a personality disorder they call themselves "disabled", but I wouldn't deem this adequate reason to have preferential treatment as regards needing a seat.
You also running the risk of insultng people, qf. offering a seat to a 'pregnant' lady who isn't, to an independent minded disabled person who would feel insulted that you judge them as unable to stand, or the decrepit looking older person (my grandfather was one of these looked frail but was tougher than old boots) who in fact could walk you off your feet and doesn't need your charity.
At the end of the day it comes down to a simple question - who are you to judge the capability of these people to sit or stand? Whether you mean to or not, by offering your seat to someone you're saying 'I don't think you're as capable as me.' Some people are very sensitive to these sorts of judgements.
As a very long time consumer of NHS services with a chronic health condition, we're strongly encouraged to take ownership of our illness. That means being vocal about your needs and how you want to be treated. It's up to you to ask, not for others to give. I have occasions when I'm unable to stand for long periods, however there's no way I'd fall into your judgement of someone who looks like they need help. In that situation, I would ask if people would mind if I sat down. Every time I've done so, someone has helped me out. It's up to you to vocalise your needs, not for you to stand there looking helpless until someone takes pity on you.
Times are changing when it comes to treating people with disabilities. We're no longer to be coddled or treated as 'different'. For example, I will stand for a woman wearing a 'Baby on Board' badge, as she's taken a positive step and identified that she would like to sit where possible. Aside from that I will wait to be asked.
I've also taken the decision not to stand for women on public transport. I used to until I realised that actually I was saying 'as a woman you're not as capable as me as a man, so please have my seat.' If I wouldn't give up the seat to a man of a similar age / build, then I won't do it for a woman.
.
