switch to the db you're using and type:
mr = db.runCommand({
"mapreduce" : "myCollectionName",
"map" : function() {
for (var key in this) { emit(key, null); }
},
"reduce" : function(key, stuff) { return null; },
"out": "myCollectionName" + "_keys"
})
once you get result, type:
db[mr.result].distinct("_id")
and you will get a list of fields names.
Get all fields names in a mongodb collection? - Stack Overflow