If it's Brut, it'll say so on the label.
You want quality? Money no object? Take a lesson from Malcolm Gluck, respected former wine critic for the Manchester Guardian...
Two years ago, towards Christmas, Asda reduced the price of their own-brand Cava to �2.49 a bottle. Gluck's opinion? 'This is better than Krug champagne selling at �100 a bottle!'.
By law, to be called 'champagne', a wine must be produced in the Champagne district in France. Cava, often from Spain, is produced in exactly the same way - you'll often see 'methode champagnoise' on the label. Because of the name, champagne enjoys premium prices, but by no means does this guarantee quality. Blind taste tests often result in cheaper Cavas getting higher marks than expensive champagnes.
Do yourself, and your bank balance, a favour.