How to look up the public key for an address?

Support FAQ
1

You can query blockchain.info:

Example

https://blockchain.info/q/pubkeyaddr/1HwpTBqdCzewurHg4z3yDfF3sH9o6qwLWe will return
0242c539be69ab257fd6bc8381a4674dbfa3fc9aeceedb78c8852dae7e76a6539c

If the address is in your wallet, you can use the bitcoind command:
validateaddress