Tell them you have a positive attitude towards LGBT people. Try to find out if they also respect LGBT people. If you find out that your friend hates/disrespects LGBT people, don't try this method with them.;
, If you are close enough, either of your houses could be the best option. Make sure that you two are alone. Don't select a place where others can invade your privacy.
 Make sure that the atmosphere is calming and that neither of you have any unrelated stress. Try to make your friend feel safe with you.
 Try to turn the conversation towards LGBT topics so that your question doesn't come from out of the blue. Then, you can either slip it quietly into conversation or ask outright.
 Be patient and understanding. Understand that they may not actually be bisexual. If your friend tells you that they are not bisexual, don't press the topic too much. Either they are actually not bisexual or not ready to come out, and in both cases, pressing them on the topic will do more harm than good.
 Their coming out to you is most likely a sign of trust, so don't betray that trust. Make sure that your language and movements are calm and friendly. Remind them that their identity is valid. Say you'll be there for them and back up your words with action.
Make sure that your friend knows that bisexuality is okay.
Invite your friend to an appropriate location.
Act casual.
Ask.
Watch how your friend reacts.
If your friend is bisexual, be supportive.
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