How can I know if the person I am in love with have my same sexual orientation?

The only way to be sure is to ask them. Maybe you can try to inquire about their views on LGBT first, so you'll know if it's "safe" to ask, or you can try to learn more about their previous partners and experiences. Just remember that whatever you find out is no guarantee, they could have a different orientation than what it seems, or they might have hidden it for some reasons. So ultimately, asking is really the only way to be sure.

Ask a friend, ask them about celebrities they think are cute, play it casual. And don't sweat it.

There is no way to fully know what another person's sexual orientation is if you don't ask them. You can try and find clues, but, in the end, the only way you will fully know, is if you ask the person yourself.

The only way to know is to ask them directly or ask people that know that person well enough. For example you can ask his/her close friends or siblings.

Well you could politely ask, or someone that knows them. Its always better to be sure than to define what someone is by the way they act or dress--after all, these things have nothing to do with that!

Honesty is the best policy. Generally, when unsure of someone's sexual orientation, simply asking is the best way to go. It may sound blunt, or brusque, but you don't want to mistakenly try to get involved with someone that may not have any interest in you in that capacity. However; specific wording and how you approach the person is something that is all on your end/in your hands as it were. It just boils down to being open and honest with another person.

Just ask them casually what sexual orientation they identify with. Or you can ask their friends about their relationship history

You can't know for sure unless they tell you, either after you've asked them about it or because they decided to tell you.

Never make assumptions about someone else's sexual orientation. That can make them uncomfortable. Ask kindly or watch who they date. Never base your opinions on rumors or hearsay.

The only good way is to ask them, otherwise you'd have to stereotype or go behind their backs and ask one of their friends