Power them on and run something to exercise the read function over every bit. Thats why a ZFS filesystem integrity check/scrub is the useful model.

I'm unsure if dd if=/the/disk of=/dev/null does the read function.

why would it not? it's a low level tool to do exactly that. you could "of" it to somewhere else if you're worried it's not. I like to | hexdump -C, on an xterm set to a green font on a black background for a real matrix movie kind of feel.