If he walks like a priest and talks like a priest, then he is a priest until you prove he isn't.
Con men who pose as priests commonly prey on the Hispanic community. Usually they are Novus Ordo and sometimes they set up small chapels. They "walk and talk" like priests and make a living by performing fake baptisms and legal marriages (any person can obtain a license to perform marriages).
That said, Wiest exhibits behaviors not seen in legitimate Catholic priests.
A priest will show his ordination papers and will say where, when and by what rite he was ordained; Wiest refuses.
A priest who asks to work alone with altar boys will show his drivers license and submit his fingerprints; Wiest refuses.
A priest does not brag from the pulpit that:
"As a little child, St. Theresa came often came to play with me."
and
"If I become a saint it will be because of my great charity."
and
"Our Lady of Fatima said the Miracle of the Sun would happen again a second time (She said not such thing) and I was privledged to see it."
A priest does not make up stories and then change them.
For months his story was
"I first met Father Perez in Rome when we were both young Seminarians and I had nowhere to go and he let me stay for a period of time."
then after several repetitions of this story he changed his story to:
"I first met Father Perez in a small town outside of Florence at Christ the King Seminary when three seminarians and myself were on on way to Sloviakia and he permitted us to spend the night."
(This revision is more believable and the details of where and when Fr. Perez was in this small town is easily available online -- too bad Wiest hadn't seen it first before he made up his Rome fairy tale.)
He has also changed his age from 48 to 58.
He also changed his story about his cancer from happening in his last year of high school to his last year of law school at Yale (and no, not a remission, just a revision of his story.)
His response to the photo of the Budda altar in his home was that his privacy was invaded. (Yes, please allow the poor man to engage in Pagan ancestor worship in the privacy of the home we are paying for.)