(assuming you can obtain access to the raw apk file - which you usually can, if you know or make an educated guess of its name and location, even though you can't list the contents of /data on a non-rooted phone)
You could open the apk as a zip file and filter the ascii text from the binary content of META-INF/CERT.RSA
Or using an actual tool,
jarsigner -verify -certs -verbose some_application.apk
Of course the only way to verify that the signer is who they claim to be is to get something else signed with the same key from that party via direct or verified means and compare the signing key fingerprints - that is how Android itself verifies that app upgrades and app ID sharing come from the same party as the existing APK they target.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…