1
An attacker controls the majority of the hashing power and wants to double spend without arousing suspicion. If they mirror the public chain's activity while generating the private fork and the fork is short (say 10 blocks), can you only detect the double spend by verifying the block hashes haven't changed or noticing the double spend transaction directly?
3
The short answer is that there is no way to double spend "without arousing suspicion". Presumably, you would be double-spending against someone and that person would notice as soon as you diverted the main chain.
longer answer: As long as you're building your private chain you can double spend everything you have by normally broadcasting to the public chain, knowing that it will be reversed later. There is no way to detect/defend against a 51% attack until it happens, and once it happens the security of the blockchain of Xcoin is obviously compromised and the whole coin/your stash of coins is now worthless, so there was not much point in the double spend anyway.
2I'm not sure I understand what you are asking. Blocks are compared by comparing their hashes, not just by looking at their transactions. If the blocks contain the same transactions but are not identical (different nonce, or reward paid to different addresses), the network will see that they are on two different forks. If they are identical, then there is no fork at all. – Nate Eldredge – 2014-04-13T21:52:38.927
I edited it a bit. I'm wondering if there's any other ways to detect the double spend attack if you don't keep a record of the primary chain block hashes prior to the longer private fork replacing it and miss the original double spend. – chizu – 2014-04-13T22:05:27.300
As this is a general question applicable to most decentralised currencies, is there any particular reason why it has been tagged [dogecoin]? It does not appear to help people looking for specific dogecoin information, and hinders people looking for general information. – trichoplax – 2014-04-14T16:24:29.533
How do you mimic a transaction? – Geremia – 2015-06-17T18:38:56.970