If she's a 'transfer maid', her employment history in Singapore should be made available to prospect employees.
Question the information the agency gives you– "If she's a good domestic helper, why didn't her employer renew her contract?"
If she's new to Singapore, it's advisable to know her situation back home so as not to cause complications in her employment commitments.
Besides checking with former employers, do also check out this group which lists all the blacklisted maids - https://www.facebook.com/groups/521151747976052/