The “currency market” on the steemit platform have features where you can see bid price of people that want to buy steem with steemdollars and also those that want to buy steemdollars with steem so it depends on the kind of exchange you want to do...

But the most official way to know the exchange rate is to visit steemd.com and you will see the price of one steemdollas per steem so for example if on steemd.con it says 1.3steemdollars per steem then it means you can convert 10steem and get 13 steemdollars after exchanging it..