How can I tell if I have a 170 or 225 s6 in my 64 Dart?

The block will be stamped (unless it was milled off in a previous rebuild) on the RF top of the block by the coil/alternator bracket. If original '64, it will be stamped V225 or V22 if it's a 225 engine or V170 or V17 if it's a 170. If there is different letter code before the engine size, it's from a different year. There are also date codes on the blocks that will show when the block was cast.