Read all the publications - Campaign, Marketing, Creative Review, Guardian (mondays) FT Creativity (Tuesday). Look at the awards books. D&AD is a good start but there are many others to look at - Campaign (posters, direct, press). Cannes, DMA, SPCA, ISP, Creative Circle, Epic. (Check out the American ones as well - not for the agencies but because some of the work is brilliant - One Show, Art Directors Club Annual, New York Festival, Communications Art Advertising Annual).

Some agencies don't enter awards or can't find categories for their more innovative approach. A hell of a lot of great work never gets to the awards (or the awards it deserves). Check out the agencies web sites. Ask around - head hunters are good. Talk to old students who've done the rounds. Find out who's done the campaigns you like (the publication ALF tells all, available in most good libraries ).

The most important thing is to make a valued objective judgement based on objective views. Don't just rely on the word of one person, no matter who they are.
The best way to decide which agencies are the ones you want to work at is to take your book in to see them (even better if you can get a placement).