[Ethereum] How to retrieve the Contract account balance in web3 or in geth command line

geth --fast has an interesting effect: geth cannot provide any information about accounts or contracts until the sync is fully complete.

Try querying the balance again after eth.syncing returns false.

Why?

The "fast" sync looks nearly complete because the reported current block is based on the best header you have. But that doesn't give you any information about how much of the account state data you have downloaded.

geth can sync the blocks much faster than it can download the state. An excerpt from @karalabe's explanation why:

This trie data structure is an intricate interlink of hundreds of millions of tiny cryptographic proofs (trie nodes). To truly have a synchronized node, you need to download all the account data, as well as all the tiny cryptographic proofs to verify that noone in the network is trying to cheat you. This itself is already a crazy number of data items. The part where it gets even messier is that this data is constantly morphing: at every block (15s), about 1000 nodes are deleted from this trie and about 2000 new ones are added. This means your node needs to synchronize a dataset that is changing 200 times per second. The worst part is that while you are synchronizing, the network is moving forward, and state that you begun to download might disappear while you're downloading, so your node needs to constantly follow the network while trying to gather all the recent data. But until you actually do gather all the data, your local node is not usable since it cannot cryptographically prove anything about any accounts.

Nonces are 0 indexed, transactions are indexed from 1. This post here mentions:

Apparently there's a nuance with the nonce... my error was I was setting the nonce to web3.eth.getTransactionCount() + 1, which left a gap between prior transaction nonce and next transaction nonce (nonces are 0 indexed, transactions are indexed from 1, that's the root of the error). It so appears that you are not allowed to increment the nonce by 2 and leave a gap. Must be sequential.

Requested instructions: You need to find out the nonce value of the first transaction you fire.

var firstNonce = web3.eth.getTransactionCount(yourSender);.

The post I linked above and here shows how to set nonce manually(and a common error, that getTransactionCount already returns a value one higher than the nonce and was the reason for my answer)