How to check Bitcoind is mining? I am still very new to Bitcoin.
I installed bitcoind on my Ubuntu server as described on bitcoin.org.

After installation I issued 


  bitcoind -daemon


and the tool started to download/get the blockchain (which is finished meanwhile). 

Three questions:


In contrast to what the setup guide says, the tool did not complain about a missing rpcuser and rpcpassword in the bitcoin.conf file. Is that an issue with the documentation or is there something wrong with my installation?
In case my node gets the chance to process a block, I would be rewarded those 25 BTCs. But to which account would the be sent? Is there a default or do I need to do some configuration for that?
As the process to become the node to mine the next block is computational very heavy, I would have expected that the CPU usage of my server should always be very high - but it's only at &lt;10%. How can I make sure that my node participates to become the one to process the next block? Is there some configuration that I missed?
Answers to your respective questions:


Since 0.12, bitcoind supports cookie file authentication. This means that on startup of bitcoind, a file with a random key in it is created. Anyone who can present the contents of that file to the running bitcoind is allowed access. Since bitcoin-cli runs locally and as the same user, it can read this file and do so.
Bitcoin does not have any concept of accounts (at least not at the protocol level). Generated funds by bitcoind's built-in miner are sent to an address taken from your local wallet. If you want to modify that, use mining software instead of the built-in miner (which is only there for testing purposes).
The built-in miner is off by default. You can enable it by setting gen=1 in the bitcoin.conf file, but I would strongly discourage you from doing so. The chance of finding a block using that setup is about 1 in a billion, if you leave it running for 10000 years. If you insist on using your CPU for mining (which won't ever be profitable), at least use optimized software like bfgminer, and sign up with a pool, so you can get small rewards for work, rather than waiting for a lottery you'll never win.


Update august 2016: The internal miner is removed entirely in Bitcoin Core 0.13.