How to check if socket is closed in Boost.Asio?

Just check for boost::asio::error::eof error in your async_receive handler. It means the connection has been closed. That's the only proper way to do this.

If the connection has been cleanly closed by the peer you should get an EOF while reading. Otherwise I generally ping in order to figure out if the connection is really alive.

Is there a boost peek function available? Most socket implementations have a way to read data without removing it from the queue, so you can read it again later. This would seem to satisfy your requirements.

After quickly glancing through the asio docs, I wasn't able to find exactly what I was expecting, but that doesn't mean its not there.

I'd suggest this for starters.