My 2p (bearing in mind I have a full time profession in the ambulance service and only lead when I can/want to - thus I'm in a different position to many leaders/instructors whose groups are their bread and butter..)
When I take a booking I send a booking form in the shape of a very easily fill in-able PDF. Tick boxes, drop downs etc.
I ask basic but valid questions about the individuals levels of general fitness - examples are:
* (score 1-5 - 1 being extremely difficult, 5 being very easy) How do you find doing 10 press ups?
* How to you find walking up a steep hill for 5 minutes?
I also ask about general fitness in terms of lifestyle, such as: 'Is your job sedentary, mildly physical, extremely physical' etc and likewise leisure time.
Coupled with their sizes which I need for kit hire, I will soon be able to pick out individuals who may not be suited to the rest of the group or even the intended trip itself.
Fortunately, I've never had anyone who couldn't complete the intended trip. I never make anyone do anything they aren't comfortable with (they are not made to feel inadequate etc for not attempting the drainpipe if they don't want to, for example).
I've only had one individual who couldn't manage the second part of the day (Swildons Hole Upper series) but the good thing was, they didn't feel bad or stupid about sitting out the afternoon. It was they who asked of it was OK to give it a miss and I was in total agreement and would have suggested it if they hadn't.
The others on the trip were a bit fitter but there are ways you can make a trip tougher for the better cavers to challenge them more when you have a weaker caver in the group.
It is not against the law to give them the bag to carry, for example - or give them a job to do - which will slow them a little. The trip should always run to the weaker caver, but there are ways of balancing the group out - you just have to think outside the box.
One thing I always do (and I'm lucky that I have the time to do this - many outdoor centres just don't) is make time to find out a little about my group beforehand and again when I meet them.
They get a little talk about caves, geology and stuff like that and they get the chance to air any concerns they have etc.
I usually have my lunch with them and listen to what they say and their mood, so I know what to expect in the afternoon when they are maybe a little tired.
This week I saw several mini busses at a cave in Wales where 3 very large ladies only managed to get to the entrance of the cave, only to struggle getting back up the hill - and that was the end of their trip. A real shame and I hope they get another chance to have a trip of their own, at their pace, taylored to them so they don't go away with bad things to say.
Everyone is capable of 'something' - but I agree with many posts here that some people just aren't designed to do some trips. This should be picked up way before they actually turn up, ideally.