[DennisP]

The competition works by increasing the difficulty. At any given time there's a maximum numerical value of the block hash; the lower it is, the more hashes you have to make before you find a valid one.

The difficulty can be checked after the fact, by simply checking the numerical values of hashes. If a hash is one in a trillion, you know it took about a trillion attempts on average to make it, and you can figure out the economic cost of doing that.

It would be easy to make a chain with lots of blocks of low difficulty. It would also be easy for anyone to see that it didn't cost much to make it.