Praise be to Allaah.
According to a hadith narrated by Anas, the prophet said,
Whoever has seen me in a dream, then no doubt, he has seen me, for Satan cannot imitate my shape. (Sahih Bukhari 9:87:123)
In their commentary on this hadith, the scholars at IslamQA state (excerpted from this answer),
…this is to be understood as referring to seeing the Prophet (peace and blessings of Allaah be upon him) in the physical form that is described in the saheeh ahaadeeth, for the Shaytaan cannot imitate the form of the Prophet (peace and blessings of Allaah be upon him).
But if the Shaytaan comes in another form, whether one is asleep or awake, then lies and says, “I am the Messenger of Allaah (peace and blessings of Allaah be upon him),” then this is false and does not count as seeing the Prophet (peace and blessings of Allaah be upon him).
To ensure that you have actually seen the Prophet (peace and blessings of Allaah be upon him), rather than the Shaytaan attempting to impersonate him with a different form, here are some ahadeeth describing his physical features (collected from here):
Rabee'ah ibn Abi 'Abd al-Rahmaan said: "I heard Anas ibn Maalik (may Allaah be pleased with him) describing the Prophet (peace and blessings of Allaah be upon him). He said: 'He was of average height, not too tall and not too short, with a pinkish colour, not very white and not dark, and his hair was neither very curly nor very straight. The Revelation came to him when he was forty years old, and he stayed in Makkah for ten years after the Revelation came, then in Madeenah for ten years. When he died, there were no more than twenty white hairs on his head and in his beard." (al-Bukhaari, 3283).
Al-Baraa' ibn 'Aazib said: "The Messenger of Allaah (peace and blessings of Allaah be upon him) was broad shouldered and had thick hair coming down to his shoulders and earlobes. He was wearing red garments. I have never seen anything more beautiful than him." (Reported by Muslim, Kitaab al-Fadaa'il, Baab Sifat Sha'r al-Nabi (peace and blessings of Allaah be upon him), no. 2338).
'Ali said: "He was neither tall nor short, and had large hands and feet. He had a large head and was big-boned, and the thin line of hair (starting from his chest and extending to the navel) was long. When he walked, he would lean forward, as if he was walking downhill. I have never seen anyone like him, before or since." (Reported by al-Tirmidhi, 3570, who said this is a saheeh hasan hadeeth).
Jaabir ibn Samurah said: "The Messenger of Allaah (peace and blessings of Allaah be upon him) was dalee' al-fam, ashkal al-'ayn and manhoos al-'aqib." Shu'bah said: "I asked Maalik, 'What is dalee' al-fam?' He said: 'Wide-mouthed.' I asked, 'What is ashkal al-'ayn?' He said, 'Big-eyed.' I asked, 'What is manhoos al-'aqib?' He said, 'His heels were not fleshy.'" (Saheeh Muslim, Kitaab al-Fadaa'il, 2339).
