The first sign: it was reported in Sahih Muslim from the hadith of Ubayy ibn Ka'b that the Prophet (peace be upon him) announced that one of its signs was that when the sun rose on the following morning, it had no (visible) rays. [Muslim 762]
The second sign: it was reported from the hadith of Ibn 'Abbas narrated by Ibn Khuzaimah, and by al-Tayalisi in his Musnad, with a Sahih isnad, that the Prophet (peace be upon him) said: "Laylat al-Qadr is a pleasant night, neither hot nor cold, and the following day the sun rises red and weak." [Sahih Ibn Khuzaymah, 2912; Musnad al-Tayalisi]
The third sign: it was reported by al-Tabarani with a hasan isnad from the hadith of Wathilah ibn al-Asqa' that the Prophet (peace be upon him) said: "Laylat al-Qadr is a bright night, neither hot nor cold, in which no meteors are seen." [Narrated by al-Tabarani in al-Kabir. See Majma’ al-Zawa’id, 3/179; Musnad Ahmad]
These three Sahih ahadith explain the signs which indicate Laylat al-Qadr.
It is not essential for the one who "catches" Laylat al-Qadr to know that he has "caught" it. The point is to strive hard and to be sincere in worship, whether or not one knows that one has "caught" it. It may be that some of those who do not know that may be better with Allah and higher in status than those who did know which night it was, because the former strove hard.
We ask Allah to accept our fasting and our prayer at night, and to help us to remember Him and to thank Him and to worship Him properly. May Allah bless our Prophet Muhammad.