How do I query referenced objects in MongoDB?

You can't. See http://www.mongodb.org/display/DOCS/Database+References

You have to do it in the client.

You can now do it in Mongo 3.2 using $lookup

$lookup takes four arguments

from: Specifies the collection in the same database to perform the join with. The from collection cannot be sharded.

localField: Specifies the field from the documents input to the $lookup stage. $lookup performs an equality match on the localField to the foreignField from the documents of the from collection.

foreignField: Specifies the field from the documents in the from collection.

as: Specifies the name of the new array field to add to the input documents. The new array field contains the matching documents from the from collection.

db.Foo.aggregate(
  {$unwind: "$bars"},
  {$lookup: {
    from:"bar",
    localField: "bars",
    foreignField: "_id",
    as: "bar"

   }},
   {$match: {
    "bar.testprop": true
   }}
)