How many coins been mined on the Testnet? How can I check how many coins have been mined?
By visiting http://blockexplorer.com/testnet you can browse the testnet blockchain.
As of this writing, it shows that 128558 blocks have been mined on the current iteration of testnet (testnet3). Since each block has a reward of 50 BTC (it has not decreased), that's a total of 6427900 BTC mined to date on testnet.