I did a google search for "test for meltdown and spectre" and found a really disconcerting collection of disreputable-looking links.
I did find an ubuntu page on these attacks which referred to what appears to be an authoritative page detailing these attacks with lots of information. However, there does not appear to be any trustworthy script that one can run to check for this vulnerability on one's machines.
Does anyone know of such a script?