Relationships are about choices, and so is consent. Consent is both you and your partner choosing to participate in sexual activity, and making that clear to each other. It's not complicated; it's just the right thing to do.
Consent isn't "They didn't say no." Consent is "They said yes." Be a real partner and do the right thing: ask.
Chances are, you're not going to formally say "Partner, do you hereby consent to...?" That sounds a little awkward. Consent can take the form of asking questions like “Can I [fill in the blank]?” or “Do you want me to do [fill in the blank]?”
Planned Parenthood offers good suggestions and some quick videos that show how couples can talk to each other.
What does the law say about consent and alcohol or drugs? There cannot be consent where the victim/survivor is “incapable of consenting to the activity”. There is, however, no definition of how drunk or high someone needs to be, to be unable to consent. This would be decided by judges/juries if a sexual assault charge makes it to court. The law is clear that all of the parties must be conscious and awake during the sexual activity, even if they consented previously. (Source: Break the Silence)