It’s not hard to know if someone is actively consenting to sex. Ask them!
Before we get going here, this is an excellent web site that explains the terms and definitions of all things related to consent: Calisto: Understand the Language of Consent.
Consent is a deliberate verbal agreement between two (or more) people to engage in sexual activity. This is a conversation to have BEFORE any sexual activity has commenced, that is before any articles of clothing have been removed!
If your date is under the influence of drugs or alcohol, you’re in iffy territory. It’s best to have made the agreement long before anyone partakes of anything. And even then, it’s important to check in with them periodically to make sure they’re willingly engaged.
“Are you sure you’re comfortable with this? I know we agreed we were going to have sex, but you drank a lot of shots, and I want to make sure you are still OK with this.”
If they confirm their intent, proceed with caution! Recognize there are risks to what you are doing.
If they’ve had a lot to drink, you already know they’re not able to make a deliberate choice. Don’t have sex. Put it off. Talk about it again when you’re both sober, like the following morning, when you’re crunching on dry toast and passing the Pepto back and forth.