Was the trust created on your Mom's dead with your Dad being the income beneficiary for life and the balance passing the beneficiaries on his death?
Normally these are set up as an irrevocable trust and tax return is filed every year.
You should request the trust documents. Normally with these kind of trust the property is distributed at the death of the surviving spouse to the named beneficiaries.
And regarding tax return- The trustees must get a taxpayer ID number for the irrevocable trust and file an annual trust income tax return-Form 1041. You can request a copy of the same from the trustees.
Let me know if you have any question.
Please note: This advice is provided with the understanding that all the relevant facts have been provided by you. Any change in facts might affect the advice given and hence may not be relied on in such cases. Nothing contained in this reply was intended or written to be used, can be used by any taxpayer, or may be relied upon or used by any taxpayer for the purposes of avoiding penalties that may be imposed on the taxpayer under the Internal Revenue Code of 1986, as amended.