All perfect praise be to Allah the Lord of the Worlds. May His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
The angels are no more than servants of Allah: they are raised high in honour, they never disobey Allah, they are created from light, and they can`t be seen by people; however, Allah may bless someone by allowing them to see the angels as they are or as they appear. Many evidences from Sharia support these rulings. For example, 'Aishah (May Allah be pleased with her) reported: The Messenger of Allah (PBUH) said, "Angels were created from light, jinns were created from a smokeless flame of fire, and 'Adam was created from that which you have been told (i.e., sounding clay like the clay of pottery)."[Muslim]. `Aisha said, "Once Allah's Messenger (PBUH) said (to me), 'O Aish (`Aisha)! This is Gabriel greeting you.' I said, 'Peace and Allah's Mercy and Blessings be on him, you see what I don't see". {Bukhari& Muslim}. Moreover, Allah the Almighty says: {What means}: " then We sent her our angel, and he appeared before her as a man in all respects." {Mariam, 17}. According to authentic narrations, Gabriel used to come to the Prophet Mohammad (PBUH) in the form of Dihya ibn Khalifa al-Kalbi or a man that nobody knew.
All this indicates that humans can`t see angels in the form Allah created them except that-as was narrated-Prophet Mohammad (PBUH), and none else, saw Gabriel twice. However, if someone claimed seeing angels in their true form and he/she was truthful and righteous then we commit his/her affair to Allah for this isn`t a ruling of Sharia.
As for seeing angels in other than their true form, i.e. in human form, this is possible and happened to many of the companions (May Allah be pleased with them). ‘Umar b. al-Khattab said: One day when we were with God's messenger, a man with very white clothing and very black hair came up to us. No mark of travel was visible on him, and none of us recognized him. Sitting down beside the Prophet, leaning his knees against his, and placing his hands on his thighs, he said, “Tell me, Muhammad, about Islam." He replied, “Islam means that you should testify that there is no god but God and that Muhammad is God’s messenger, that you should observe the prayer, pay the zakat, fast during Ramadan, and make the pilgrimage to the House if you have the means to go." He said, “You have spoken the truth."……………….. [‘Umar] said: He then went away, and after I had waited for a long time [the Prophet] said to me, “Do you know who the questioner was, ‘Umar?" I replied, “God and His messenger know best." He said, “He was Gabriel who came to you to teach you your religion." {Bukhari&Muslim}.
In addition, narrated Sa`d bin Abi Waqqas: I saw Allah's Messenger (PBUH) on the day of the battle of Uhud accompanied by two men fighting on his behalf. They were dressed in white and were fighting as bravely as possible. I had never seen them before, nor did I see them later on. {Bukhari}. Sa`d reported that on the Day of Uhud I saw on the right side of Allah's Messenger (PBUH) and on his left side two persons dressed in white clothes and whom I did not see before nor after that, and they were Gabriel and Michael (Allah be pleased with both of them). {Muslim}. And Allah the Almighty knows best.