How does the practice of "Urim and Thummin" in the Old Testament differ from forbidden practices of divination?

We do not totally understand how Urim and Thummin worked in the Bible. We know they were objects that fit into a pouch of the breastplate worn by the high priest (Exodus 28:30And thou shalt put in the breastplate of judgment the Urim and the Thummim; and they shall be upon Aaron's heart, when he goeth in before the LORD: and Aaron shall bear the judgment of the children of Israel upon his heart before the LORD continually.
See All...; Leviticus 8:8And he put the breastplate upon him: also he put in the breastplate the Urim and the Thummim.
See All...). We also know that they were a way to ask counsel of the Lord (Numbers 27:21And he shall stand before Eleazar the priest, who shall ask counsel for him after the judgment of Urim before the LORD: at his word shall they go out, and at his word they shall come in, both he, and all the children of Israel with him, even all the congregation.
See All...). Their use was restricted to the priest. After the return from Babylonian captivity, some priests who had a questionable genealogy had to wait for a priest with Urim and Thummin in order to determine their ancestry (Ezra 2:63And the Tirshatha said unto them, that they should not eat of the most holy things, till there stood up a priest with Urim and with Thummim.
See All...; Nehemiah 7:65And the Tirshatha said unto them, that they should not eat of the most holy things, till there stood up a priest with Urim and Thummim.
See All...). We do not know if that priest ever showed up.

Other references speak of the ephod and indicate it included a way to get counsel from the Lord (1 Samuel 23:6-12 [6] And it came to pass, when Abiathar the son of Ahimelech fled to David to Keilah, that he came down with an ephod in his hand. [7] And it was told Saul that David was come to Keilah. And Saul said, God hath delivered him into mine hand; for he is shut in, by entering into a town that hath gates and bars. [8] And Saul called all the people together to war, to go down to Keilah, to besiege David and his men. [9] And David knew that Saul secretly practised mischief against him; and he said to Abiathar the priest, Bring hither the ephod. [10] Then said David, O LORD God of Israel, thy servant hath certainly heard that Saul seeketh to come to Keilah, to destroy the city for my sake. [11] Will the men of Keilah deliver me up into his hand? will Saul come down, as thy servant hath heard? O LORD God of Israel, I beseech thee, tell thy servant. And the LORD said, He will come down. [12] Then said David, Will the men of Keilah deliver me and my men into the hand of Saul? And the LORD said, They will deliver thee up.
See All...; 30:7-8
See All...). It is very likely that the ephod had the Urim and Thummin with it. Other examples of revelation that probably came by way of Urim and Thummin are the idetifying of Achan (Joshua 7:16-20 [16] So Joshua rose up early in the morning, and brought Israel by their tribes; and the tribe of Judah was taken: [17] And he brought the family of Judah; and he took the family of the Zarhites: and he brought the family of the Zarhites man by man; and Zabdi was taken: [18] And he brought his household man by man; and Achan, the son of Carmi, the son of Zabdi, the son of Zerah, of the tribe of Judah, was taken. [19] And Joshua said unto Achan, My son, give, I pray thee, glory to the LORD God of Israel, and make confession unto him; and tell me now what thou hast done; hide it not from me. [20] And Achan answered Joshua, and said, Indeed I have sinned against the LORD God of Israel, and thus and thus have I done:
See All...) and the refusal of God to answer Saul (1 Samuel 14:36-37 [36] And Saul said, Let us go down after the Philistines by night, and spoil them until the morning light, and let us not leave a man of them. And they said, Do whatsoever seemeth good unto thee. Then said the priest, Let us draw near hither unto God. [37] And Saul asked counsel of God, Shall I go down after the Philistines? wilt thou deliver them into the hand of Israel? But he answered him not that day.
See All...; 28:6
See All...).

You question about distinguishing Urim and Thummin from forbidden practices of divination is difficult because of our ignorance on how it worked. I used to think that it was just some way to get yes and no answers from God until I realized that some of the answers they received were not yes and no answers. God could also refuse to answer by Urim and Thummin. The main distinctions I see is that the Urim and Thummin was established by God to be used only by the priest and that its counsel had the same authority as other forms of revelation--it was always absolutely true. In contrast, any form of divination established by man is an abomination to the Lord (Deuteronomy 18:10There shall not be found among you any one that maketh his son or his daughter to pass through the fire, or that useth divination, or an observer of times, or an enchanter, or a witch.
See All...; 2 Kings 17:17And they caused their sons and their daughters to pass through the fire, and used divination and enchantments, and sold themselves to do evil in the sight of the LORD, to provoke him to anger.
See All...; Acts 16:16And it came to pass, as we went to prayer, a certain damsel possessed with a spirit of divination met us, which brought her masters much gain by soothsaying:
See All...).