Potterfan, is this for choosing A level subjects for some time in the future, or trying to decide on a third subject to start in year 12 next week?
If the former, then they have got another couple of months at least to seek out the relevant teachers at school and discuss what is involved in each course. S/he can also find the specification specification or the relevant exam board online and have a good read of that. And / or find a friendly sixth former studying the subjects and ask them?
If it's next week we are talking about, reading the spec online is probably the only one of those your DC can reasonably do between now and then
FWIW, DD takes Politics (just going into year 13); she became interested in the subject, both as an academic discipline and IRL after being encouraged by a friend at school to get involved with our local Youth Council.
Based on her grades across year 12 and in her end of year exams, she hopes tbe predicted (and to achieve, of course

) an A. (But ditto French and Eng Lit, so I don't know whether Politics is meant to be an 'easy' option).