How do I check a package's signature in Linux?

[petulia3478]

# /bin/rpm -K --nosignature quota-3.13-1.2.3.2.el5

For more detailed output, add the verbose switch:

# /bin/rpm -Kvv --nosignature quota-3.13-1.2.3.2.el5

If your rpm is not corrupted, you should see:
package_name: md5 OK