db.currentOp() doesn't give information that allows an admin to see what particular collection and what field is being indexed:

> db.currentOp()
{
"inprog" : [
{
"opid" : 60013,
"active" : true,
"lockType" : "write",
"waitingForLock" : false,
"secs_running" : 23,
"op" : "insert",
"ns" : "test.system.indexes",
"query" : {

},
"client" : "172.26.190.17:45019",
"desc" : "conn",
"msg" : "index: (1/3) external sort 1824303/150037316 1%"
}
]
}