I would like to see what a PETH on Kovan is worth in DAI. How can you look for exchange rates on testnets?
There are no exchange rates for test networks as none of the assets are worth anything. They are all valued at zero.
You can get free Ethers from a testnew faucet and use those to exchange to any other tokens. The exchange rate may be basically anything - it makes no difference whether you pay 1 Ether or 1 wei to get your tokens as you can always get more free Ethers. And probably there are free token faucets as well, at least for some tokens.