We can be sure because this is the block with more than 5 million days destroyed.

Most DGC blocks only contain the coinbase transaction and have a zero or low amount of days destroyed as you can see from the attached image.

The highlighted parts show much larger than usual days destroyed numbers.

You can quickly scroll through this page by page at dgc.blockr.io if you want.

It doesn't add up to 560k, it adds up to 646,081.21490442 DGC

It's not outside the realms of possibility that whoever did this also transferred some other coins from somewhere else to dump at the same time or someone else also did a transaction which was included in the same block - this is a common phenomenon and is why we have a distributed block chain.