Unless the key is obtained direct in person from the owner of the key one can never be certain as to the authenticity of the key. Two methods exist to help verify the public key. All keys are signed, or at least they should be. If you have a key from one of these third parties you will be able to verify the key. Each key has a unique 128 bit digital fingerprint.
Many public keys are obtainable from public key servers.
No checks are made on whoever is uploading the key, the key server itself could be attacked, the communication channel is not secure. The keys should always be regarded as untrustworthy and subjected to verification. PGP can be used to sign messages. This is the same process as used to sign a key. The presence of a digital signature can be used to verify the authenticity of a document or file.