Assalam Valaikum,
Kindly answer my below questions (are somewhat weird ) :
1) What is the difference between an intersex person and a transgender ?
Is there any sin upon them ?
2) I nowadays see many girls engaged in haraam relationships with boys. If in case a boy is looking for a bride, how can he know if that girl is a virgin ?
I know that the boy may ask her, conduct virginity test on her or may put a condition on the marriage contract concerning her chastity. But according to some customs

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu 'alayhi wa sallam, is His slave and Messenger.

A Khuntha (hermaphrodite) is a person who has both male and female genitalia (a penis and a vagina). Such a person was created by Allah, The Exalted, in this state and, therefore, there is no blame on them, and they will not be held accountable for it.

As for a transgender, the ruling on their situation entails detailing. A transgender person may be committing a sin (by changing gender) in certain situations and may not be considered sinful in others. A transgender person is sinful if there is no Sharee‘ah-acceptable reason to undergo a sex-change operation. However, if medical examinations prove that this person has female-typical anatomy on the inside (internal reproductive organs) but has male external genitals on the outside or vice versa, then there is nothing wrong with undergoing a sex-change operation (gender reassignment surgery).

This is as far as the first question is concerned.

As for the second question, the basic principle is that a girl is considered a virgin in the sight of the Sharee‘ah. When a man intends marriage, he should seek a righteous and well-mannered girl from a decent family, since it is more likely that such a girl would distance herself from illicit relationships. As you mentioned in the question, it is permissible for a man to stipulate virginity as a condition in the marriage contract. If he later discovers that his wife's hymen is broken (after concluding the contract), he is given the choice to annul the marriage. It should be noted that a broken hymen does not necessarily mean that a girl has committed Zina (fornication), because a hymen may break due to many reasons. Shaykh Ibn ‘Uthaymeen said: “A woman with a broken hymen does not mean that she is corrupt (i.e. committed illegal sexual intercourse); it may occur due to reasons other than sexual intercourse, such as heavy menstrual bleeding, a (violent) fall, a sudden jump, or the like. This does not mean that I am encouraging girls to act imprudently. Rather, I want to eliminate the doubts that a husband may experience in such a situation.” [Fataawa Noorun ‘ala Ad-Darb]

Therefore, if the girl is religious and well mannered, he should keep her as his wife and not divorce her merely because he discovered that she had lost her hymen.

As for testing to check whether or not her hymen is broken, he has no right to ask her to undergo such a test because it entails exposing her major ‘Awrah (private parts), which is impermissible except for a (Sharee‘ah-acceptable) necessity, as scholars mentioned.

Allah knows best.