You can spot one if it looks like this:
https://ipfs.pics//ipfs/QmXMVcQcBPKUXa7AAYMVcnhhUfp3wMv7YEDr5rhedvBs6X
https://steemit.com/steemit/@deepsynergy/steem-whale-symbol
On the more serious note, the whale is someone who has accumulated a lot of STEEM POWER compared to other users in the blockchain. Often they are early adaptors of the Steem network.
RE: What is a whale, anyways?