Is there a way to verify stock ROMs?
Yes, you can check the SHA1 or MD5 checksum.

You didn't specify which ROM you are downloading. Let's have a look at the Nexus stock ROMs:

https://developers.google.com/android/nexus/images#mantaray

There is a download link and a version number and checksums listed like that:


Version: 4.2.2 (JDQ39)
Link: https://dl.google.com/dl/android/aosp/mantaray-jdq39-factory-d79f489e.tgz
MD5: b7a1162fb4e617143306ef6c4ca6c040
SHA1: d79f489e1001d183b31d8a407b47cd5b8e9505cd


To verify a checksum you can do it via command line:

$ md5sum mantaray-jdq39-factory-d79f489e.tgz
b7a1162fb4e617143306ef6c4ca6c040


Or:

$ openssl sha1 mantaray-jdq39-factory-d79f489e.tgz 
SHA1(mantaray-jdq39-factory-d79f489e.tgz)= d79f489e1001d183b31d8a407b47cd5b8e9505cd


If it matches with the one stated at the google page, you can assume the file is not corrupted or modified.

Note: Linux, MacOS, Windows

The terminal commands only work out of the box in MacOS or Linux. To verify checksums in Windows there is a tool by Microsoft called Checksum Integrity Verifier.