As someone in that profession, it’s very hard. Probably the best way is by personal recommendation – can any of your friends recommend anyone?
The directories can also be a good ‘general’ guide for law firms that are top of the pile in a particular area – they are Legal500 and Chambers and Partners, but bear in mind that firms pay for them, so other than perhaps getting an idea of which firms do a particular type of work, take with a pinch of salt (particularly the individual recommendations – I know from working with people in those directories that what it says they’re good at and what they are actually good at are two different things).
It also depends on what service you want – a diddly high st firm is probably going to be fine for residential conveyancing, but struggle if you want a highly-leveraged corporate finance deal with offshore elements, for example. Location is also a factor – generally there are better firms in larger cities.