By sitting down with all the bills and combing with a fine tooth.
Turns out I've been paying £5 every month to BT for Their cloud option, never knew existed, never asked for it. It's set up in my DH's name, the bills went to his emails, I just saw that instead of £17 the bill was never under 25. So we cancelled that and got a 6 months refund as well.
And the water bill, they wrote to me back in september to say theat they will increase the bill to £60. Out of the blue with no justification. From £18. I called back then straight away, they amended their decision, I was happy.... but guess what, they continued to take £60!!!!! I called back yesterday, oh, sorry, our bad, you only need to pay £15 a month, but why didn't you check, he goes, it's your money!!!! errrrmmm, because I did, we had a discution, we're all grown ups, I'm paying you for water, not baby sitting!
So quite a hefty refund coming back.
So, the moral of the story is: take 6 minutes and check your bills. I do it every 6 months, looks like I need to check every other month.