How can you tell a block is Classic vs Core? Sites like http://xtnodes.com/ are tracking which versions of Bitcoin software are being used to create blocks. Where are they looking in the block structure to determine which software version generated the block?
Ah, not surprisingly, it's in the block's "version" field. It looks like core blocks are versioned as "v4", while "classic" blocks are versioned as "v805306368" and "BIP109".