I'd think a blood test would be the only accurate way of being sure. But whether that would determine whether you ovulated now, or whether it would have to be your next cycle I'm not sure.
I'm on my first cycle of clomid, and had a BT test today (day 21) to see if I ovulated.

Seems a little funny though that your AF hasn't been late before and now it is. My cycles have generally always been a little whacky and long, so it wouldn't be out of the ordinary for me to be annovulatory iykwim. As for whether or not to worry ... I think it never hurts to see your GP if you're concerned.