The only way you can be certain someone has given their consent is by them saying yes freely and willingly. Flirting, kissing and engaging in other forms of sexual activity does not mean someone has consented to all forms of sexual activity.
If someone invites you back to their home or hotel room, or if they come to yours, it doesn’t mean they are automatically agreeing to sex or any form of sexual activity.
For sexual activity to be considered consensual, the person must give consent freely and willingly and be physically and mentally capable of giving consent.
Things which may affect somebody’s ability to give consent are:
The person initiating sex is responsible for getting consent. They need to ask themselves if the other person is capable of giving consent.