Check the footer of the sites, there is usually some indication there, or at least a theme design or designer, then you can trace that to what it was built for.
I don't think there's any tool for this. Try to search for something in the footer of the site and you might get lucky . If you're familiar with a number of CMS then recognising one should not be a problem. Else, just post the link here and I'm sure someone will tell you the CMS.
First, look at the footer. If it doesn't identify the CMS, then look at the source and check the meta-tags for any identifying remarks. If that doesn't help, search the entire source for any info. If none of that works, then email the webmaster and ask.