Question

I think explain() will tell any possible index it can use. How about just showing all the indexes defined on the collection? (or even for the whole db?)

Answer 1

>db.system.indexes.find();

>db.system.indexes.find( { ns: "tablename" } );

will give you something like

 { 
  "ns" : "test.fs.chunks", 
  "key" : { "files_id" : 1, "n" : 1 }, 
  "name" : "files_id_1_n_1" 
 }

for every index (ns is the collection name).