If I see there was less than 20% down at purchase (without a piggyback) or if I see the servicer changing since the loan was issued, I assume there is MI either paid for by the seller or added by the new lender when they bought the loan.
1st, I call the servicer and ask, REPEATEDLY, because you have to find the rep that hasn't got them memo not to give out this critical information. Get the MI company and the mortgage certificate number (it's like the account number for MI).
2nd, I also ask for it in a qualified written request (search for that on this site if you don't know what that means). Remember to ALWAYS get your QWR at listing, because the bank has three months to respond.
3rd, You might even ask the mortgage broker / loan officer that gave them the loan, if the MI was put on because the downpayment was below 20%.
4th, Last but not least, when the negotiator starts blaming the mortgage insurer for the problem, I ask them "which MI company?" Usually, even at a bank, blaming a ghost company seems a little rediculous and I get the info I've been asking for all along.
I suppose if completely thwarted you could fax in a LOA to each of the companies and call down the list until one of them admits they're insuring this loan.