Jim Mallinder, the Northampton rugby director, believes next Sunday's return clash in Cardiff will be the true measure of his Aviva Premiership leaders after they beat the Blues 23-15 in their Heineken Cup Pool 1 clash at Franklin's Gardens.
England star Chris Ashton and fellow winger Paul Diggin showed their finishing power with the tries which enabled Northampton to maintain their winning run in the competition, while fly-half Stephen Myler kicked the other 13 Saints points.
"It wasn't our very best performance but we won a hard-fought game. It was a job well done but probably not a lot better than that," said Mallinder.
"Cardiff are a very good team with some good international players and next week will be a real test of where we are as a team.
"To go down to Cardiff and, if we can, get a win there, will be a good sign of our growing maturity."
Cardiff had prop Tau Filise and hooker Gareth Williams, involved in a late dust-up with Dylan Hartley, sin-binned leaving their coach Dai Young questioning the first of his side's yellow card.
Filise went for a ruck offence two minutes after Saints centre James Downey was dispatched for a high tackle on Cardiff winger Tom James. Young said: "That was harsh. I think there was a bit of evening up there because I didn't see what he did."