Now I don't use Clonezilla, but according to this page about Clonezilla Live
there's an option to calculate checksums for the images:
-gm Generate image MD5 checksums
Causes Clonezilla Live to calculate MD5 checksum(s) of image(s) created. If the image gets corrupted afterwards, the checksum allows to notice the corruption before the image is restored. Mind you, calculating the checksum takes some time and slows the process down a little.
I gather you should be able to do this when creating the images with it. It would be good if it could produce you two checksums, one for your *real* system (at the time of cloning) and one for the created image, after which you could verify that they are identical to see that the backup is fine. I can't say if Clonezilla can do that, and if it can't, your only means of checking whether the cloning operation really did work without problems is to put it on a 2nd disk and see if it works, like btncix mentioned. But you should still calculate a checksum of the image, for then you could later compare if the image is still exactly the same as when it was created, or if it has changed for some reason (when you copied it to another place, for example). Calculating checksums of big disk images takes time, but I assume it's still faster than laying out the image onto a disk and booting that. And it's pretty reliable, because the algorithms are made so that a small change in the data causes huge changes in the calculated checksum.
Some distribution installers use this method (i.e. give you a menu option in the startup screen) to check if the installation disc is all right before proceeding to the actual installer. That way the user can (if hardware failures during installation are not taken into account) be pretty sure that they've downloaded and burned the installer without problems and can rest assured that it doesn't fail because of some bits being wrong. Exactly what you'd like to see here, right?