cashing a check

Yes.  If you have been appointed by the court (or court officer) as the personal representative of the estate, that is one of your powers, you can sign for checks payable to your father or to his estate.

You should probably talk to an attorney about how to deal with the foreclosure.  Until you do that, I would be very careful about taking money out of the estate for anything other than strictly estate business (your father's funeral expenses, and the costs of the estate itself, which can include the legal fees), so that the mortgage company can't try to get money out of your personal pocket claiming that you've mis-used the funds it should have been able to reach for the deficiency if, like so many properties, your father's condo won't bring enough at the sale to cover the mortgage.

BB's advice is correct but perhaps incomplete. So long as the estate is in probate, the funds belong to the estate and not to you, personally. The check in question and any other funds you are able to gather in the course of administering the estate must be deposited in to an account in the name of the estate. You should not commingle estate funds with your own and you should not take funds out of the estate, as BB notes.

If you don't have an attorney representing you in the probate matter, at least consult a local attorney and get his guidance and advice as needed to handle the estate. Having opened the probate case you will have to ask the court to end the process, authorize you to distribute any assets in the estate, and discharge you as executor or administrator once you have settled all of your father's financial affairs.