This post is a bit of an aide-mémoire for myself. If you ever need to see which key signed an APK (for example to compare to a client ID in the API console when implementing Google+ Sign-In) you can actually extract the cert from the APK, and test it.
First you need to unzip the APK:
You're going to see a bunch of files extracted, including a CERT.RSA, which is usually in META-INF. If you use an alias for your key, it'll be THAT-ALIAS.RSA.
You can then output the signatures for the certificate with the keytool app:
This will print out the various fingerprints, and let you know the details of the certificates owner - handy for checking whether it was accidentally signed with a debug key (which will look something like this):
Issuer: CN=Android Debug, O=Android, C=US
Serial number: 4f963ac8
Valid from: Wed Apr 27 12:43:33 BST 2012 until: Fri Apr 20 12:43:33 BST 2042
Signature algorithm name: SHA1withRSA