If you wanted to know how different charities use the money they're given, eg. what proportion goes on pay/projects and so on, how would you find this out?

Maybe via a freedom of information request?

Aye that would work, although I've found that a lot of them publish their annual reports on their websites which would no doubt have such information in somewhere.

I suppose I was really wondering if there was some sort of league table or comparison source that compared lots of different ones.

I'd imagine that it would be nigh on impossible to produce a league table of the sort that you imagine.
How would you define the metric as to what counted as money going to 'charity' vs. money for staff/administration. Take for instance the Samaritans, pretty much all their money will go on staffing, administration and IT, so where would it fit in the league table?
I do take your point though, I'm just not sure what method can be reliably used to determine which charities are fleecing you and which ones are making your donations work well.

Work on the following assumptions:

More of your donation goes to fund the charity bureaucracy than gets sent to the intended recipients

More of what is sent to the intended recipients gets diverted to arms purchasing and general warlordism than gets to the aid workers on the ground

The aid workers on the ground corruptly divert more of what gets to them than they release to the intended recipients

Among the intended recipients, gangs of young males get more than mothers with starving children

And you think your ten quid makes any difference?

Blimey! I never realised the Woodland Trust was so diversive 🙂

I never realised the Woodland Trust was so diversive

A lovely neologism: diversive = subversive, in a lot of different ways

You can download a copy of a charity's accounts from the Charities Commission website.

That will tell you what they got and what they spent it on.

Cheers will have a look there

The vast majority of charities wouldn't be covered by the Freedom of Information act so you wouldn't be able to get information that way.