Ya there are actually. Usually when the garbage is really keyword dense and is junk, that is one way you can know. Another way is if the ads are blended too much with the content. These are just some basic information on how to identify an MFA.
Google has a little handbook book that Google AdSense auditors use for identifying MFA sites. However, there are millions of MFA sites and it is not possible for Google to audit all the sites using human power.
It can be easily detected by using a little script to find no of adsense ads in a page.. But still some of the MFA's can't be detected...