How to know which storage engine is used in mongodb?

Easiest way to find the storage engine being used currently in from mongo console.

Inside mongo console, type (You might need admin access to run this command)

db.serverStatus().storageEngine

If It returns,

{ "name" : "wiredTiger" }

WireTiger Storage engine is being used.

Once it is confirmed that wiredTiger is being used then type

db.serverStatus().wiredTiger

to get all the configuration details of wiredTiger.

On the console, Mayank's answer makes more sense.
On the other hand, by using MongoDB GUI like MongoChef or Robomongo storageEngine may be found by using the ways below;

On Robomongo;

---

On MongoChef;

You can detect this via:

db.serverStatus().wiredTiger 

So at "present" where this "exists" then there is a different storage engine configured other than the default "MMAPv1" where "WiredTiger" is not used.

This applies to the present "MongoDB 3.0x" series