When I do research on law firms that I'm interested in getting a training contract with, I find it hard to discern what it might really be like to work for them. How can I get a realistic picture?

Im afraid theres no substitute for actual first-hand experience. Formal vacation placements are featuring more and more in firms recruitment procedures, precisely because of the importance placed on you getting a look at the firm, and the firm getting a look at you. If you cant manage to secure a formal vacation placement (and competition can actually be fiercer than for training contracts), then see if you can arrange some informal work experience, voluntary work, or work shadowing. If youre still struggling, then try to attend open days and law fairs as chatting to existing trainees and employees will help you build a picture of the firm. Some law schools also run mentoring schemes to help students with no or limited contact with the profession to gain an insight into what working as a lawyer is really like.