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.