How to know if bitcoind synced? I installed the bitcoind and started it as daemon.
After 10hours I've tried "du -h"

ubuntu@ip-172-31-37-93:~/.bitcoin$ du -h
16K     ./database
59M     ./blocks/index
29G     ./blocks
646M    ./chainstate
30G     . 


How to know if it synced or not?

UPD found an interesting script to monitor node sync status How to check Bitcoind block chain download progress level
You can compare the block count from Blockexplorer with your local block count. Something like this:

$ wget -q -O- https://blockchain.info/q/getblockcount; echo
359721
$ bitcoin-cli -conf=/u0/bitcoin/bitcoin.conf getblockcount
359721

As you can see above my node is synched since the counter is equal.