Agreed with whats already been said..
You'll know just by touching the paint if it needs clayed.. if the car looks gleaming, but feels other wise then its time to clay!
Bear in mind that once you've clayed, you'll need to get polish/wax/sealant (of your choice) onto the paint to protect it, otherwise the time spent claying will have been in vain.
I swear by the AutoBrite product range, but thats just me.. everyones got their fav products