load server with something like this:
for (var i=0; i< 1000000;i++) {var r = Math.random(); db.foo.insert(

{i:i, r:r, rr:r, s:s}

)}
db.foo.ensureIndex(

{r:1}

)

test with this:
r=-1
db.foo.find({},

{rr:1}

).sort(

{r:1}

).forEach(function(z){ if (z.rr <= r)

{printjson(z);print(r)}

r= z.rr})

you will see one printout each time mongos switches shards