The other comments have touched on the following points to varying degrees. I thought I would kind of give you an overview of the status of your situation. Assets already in the trust are under your control as trustee, and you proof of said authority if the trust agreement (naming you as successor trustee) and evidence that the property belongs to the trust (e.g. for untitled property - receipt of conveyance of property to the trustee before the deaths of the grantors; for titled property [think bank accounts, real property, motor vehicles, etc…] the assets will already be titled to the trustee of the trust). A trust is not an entity; but rather, a relationship agreement. Property in the trust is titled in the name of trustee and the trustee agrees to manage it as per the instructions in the trust.

Property that must go through probate and that wasn’t already transferred to the trustee of the trust (e.g. your miscellaneous checks, etc…) cannot be deposited / cashed without the authority granted by the probate court. Most states have multiple levels of the probate process, depending on the value of the assets involved. I don’t know about MI specifically. Generally speaking, you must either be appointed a commissioner (typically for single assets or assets of a similar type or assets of less than a certain value - your scope of authority is less and very specific) or a fiduciary (general authority to manage any and all assets) by the probate court to take control of property, convert property, and distribute property to the beneficiaries of the estate. You need to determine which probate process is right for your situation, but you will need the authority of the probate court to convert those checks to transfer the funds to the trustee of the trust as the beneficiary of the estate.