You won't be able to see a double-spend in the block chain, because it is invalid. You will see one of the transactions included in the chain though. Blockchain.info has a page showing the most recent double-spends.
Double spend is just two (or more) transactions claiming the same input, so they will look like an ordinary pair of transaction. One of them will get included in a block, the other will eventually be forgotten by the network. A conflicting transaction to the one included in a valid block can never become a part of another block (at least until the first block will become invalidated by a 51% attack).
The standard client will not flag any invalid transactions, but it will reject any transaction that conflicts with a known transaction. It will, however, accept a transaction that is a part of a block, even if it conflicts with a 0-confirmation transaction that you know of.