Are you saying demons work miracles?
Absolutely yes by your very own Bible
Exodus 7:11 New Revised Standard Version Catholic Edition (NRSVCE)
11 Then Pharaoh summoned the wise men and the sorcerers; and they also, the magicians of Egypt, did the same by their secret arts.
Deuteronomy 13:1-3 New Revised Standard Version Catholic Edition (NRSVCE)
13 If prophets or those who divine by dreams appear among you and promise you omens or portents, 2 and the omens or the portents declared by them take place, and they say, “Let us follow other gods” (whom you have not known) “and let us serve them,” 3 you must not heed the words of those prophets or those who divine by dreams; for the Lord your God is testing you, to know whether you indeed love the Lord your God with all your heart and soul.
Matthew 24:24 New Revised Standard Version Catholic Edition (NRSVCE)
24 For false messiahs and false prophets will appear and produce great signs and omens, to lead astray, if possible, even the elect.
Mark 13:22 New Revised Standard Version Catholic Edition (NRSVCE)
22 False messiahs and false prophets will appear and produce signs and omens, to lead astray, if possible, the elect.
2 Thessalonians 2:9 New Revised Standard Version Catholic Edition (NRSVCE)
9 The coming of the lawless one is apparent in the working of Satan, who uses all power, signs, lying wonders,
Revelation 16:14 New Revised Standard Version Catholic Edition (NRSVCE)
14 These are demonic spirits, performing signs, who go abroad to the kings of the whole world, to assemble them for battle on the great day of God the Almighty.
One must be careful here to agree upon a definition of "miracles" before proceeding further. In brief:
Scripture plainly states that seducing spirits can perform wondrous acts in their dark plan of damnation, however these "wondrous acts" are not and can not be understood to be creative in their end result.
Given that later caveat, we can see that the miracles attributed to various Saints' intercession over the centuries can not have come from the Devil. Again, because he along with all his minions, can not create one single thing. Only God, who is Love, creates.
Thus, any miracle that actually "creates" something, which at least most that we commonly refer as "miracles" have done, such as a healing, or the Miracle of the Lanciano flesh, are indeed assuredly from God.
The cases where "mere" (as if that adjective truly is appropriate but for lack of a better one it will suffice) visions of Christ and/or Mary appear could, in theory be argued as demonic, if present only by themselves, especially if said visions allegedly tell the viewer to do something contrary to His Word.
However in all cases where the Church has said such are authentic, other creative miracles have also occurred in the life or due to the intercession of said Saint, so thus again, could only have come from God for the same reason as above.