Hello Justionios (or is it nikos),
I have to say that I am socked by your post. I believe that you may be new to the faith so you are eager to understand everything at once.
You are asking whether the material cross has some magical power that can be used by someone to practice blessing over the water in order to transform it to Holy Water ?
My brother watch out these teachings. They claim that the Priest as a person is using a material tool to bless Holy Water, regardless if this tools is a cross or his hand. Cross is not a tool having mysterious powers by Itself . Only by the reference to a person that presents his christian faith through the Cross can the power of Cross be manifested. And this power originates from the use of Cross by Christ so that death be defeated. In Orthodox Church there is an Eastertide hymn that says "Christ has risen from the land of death, having stepped over death by death itself". This is the real power of Cross.
You have suggested that a phoney priest may use a Cross to make Holy Water. This is not true. It can not be done by using tools.
The water is transformed into Holy Water by the Holy Ghost. Priests just pray to the Holy Gost along with the congregation for the comming of the Holy Ghost in the scene at the specific time and place in order for them to be blessed by Him. The water is transformed (not "made") into Holy Water in order for the bodies of the faithfulls to receive blessing along with their souls forasmuch as man is whole only if he is apprehended as being that has body and soul undivided.
A Orthodox priest says: "This water is blessed by the Holy Ghost", "You are married as man and wife (by His Spirit)", "this thing/person is blessed by the Holy Ghost". Everything in Orthodox Church is blessed, sanctified,consecrated, hold sacred by His Grace and Holiness and Sanctity: the Holy Ghost. The priest is a member of the congregation and in behalf of them and with them asks for His Presence to be among them. Everything is actually and literally performed in the Church by Holy Ghost. (Catholic priests are saying: "I bless this water", "I marry you man and wife", "I bless this thing/person" falsy professing that they as humans in the power of being the successors of Christ are sending the Holy Ghost according to their own blessed wishes)
Finaly, you are asking for a proof that will winnow a real priest from a pseud-priest. There is only one test: a true priest is providing the peace that has in his heart to others, to everybody not just to christians. So a true priest is giving peace to your own heart too. You shall feel it inside you. Please my brother do not search for outwardly signs because what you are looking for can only be found inside yourself.
"The Reign of God" is inside you. The truth rests there.
Lord Christ said : Matthew 23
1 Then Jesus spoke to the crowds and to His disciples,
2 saying: "The scribes and the Pharisees have seated themselves in the chair of Moses;
3 therefore all that they tell you, do and observe, but do not do according to their deeds; for they say things and do not do them.
4 They tie up heavy burdens and lay them on men's shoulders, but they themselves are unwilling to move them with so much as a finger.
5 But they do all their deeds to be noticed by men; for they broaden their phylacteries and lengthen the tassels of their garments.
6 They love the place of honor at banquets and the chief seats in the synagogues,
7 and respectful greetings in the market places, and being called Rabbi by men.
8 But do not be called Rabbi; for One is your Teacher, and you are all brothers.
9 Do not call anyone on earth your father; for One is your Father, He who is in heaven.
10 Do not be called leaders; for One is your Leader, that is, Christ.
11 But the greatest among you shall be your servant.
12 Whoever exalts himself shall be humbled; and whoever humbles himself shall be exalted".