How should dance venues best protect the drinks of attendees?

Lids and straws. Presumably this would make slipping a drug in way more obvious.

Plausibly Rohypnol and GHB are on the list of substances bees and wasps can be trained to detect. Unlike drug-sniffing dogs, bees and wasps can only be trained on a single scent and are less influenced by their handlers, so it's less of a privacy invasion and you can worry less about accidentally detecting drugs you don't mind. My impression is the bees need to be in a cylinder that's held fairly close to the source of drugs, but I really really want your friend to release trained roofie hunting bees into clubs where they swarm rapists, Jupiter Ascending style.

As far as I know, the only real solution to this and other such problems is hiring better bouncers - with good situational awareness, good judgment of people, good ability to become very present/intimidating or very invisible/unobtrusive at will. Have some at the door, turning away people who give the wrong vibe, and some inside the venue, making the rounds in a friendly way.

Also, please don't listen to the more outlandish suggestions here (ban drinks, ban drinks on the dancefloor, have gatekeepers between parts of the venue, tape lids on drinks, post scary signs). These will actively turn off good people. The venue should feel like a place of free-rolling comfort and joy.

Have video surveillance, from cameras in positions that people can't readily determine, and a known, credible commitment to review footage when incidents are reported.

Test strips/straws will have both false positives and false negatives, both of which mean they don't work for routine use--false positives would lead to panic and false accusations, false negatives mean someone could take one home and test different drugs until they find one that it can't detect. OTOH it might make sense to stash a few behind the counter and take them out when there's concrete suspicion.

If any drink with juice does trigger the reactive straws the bartenders can learn about which drinks do that and then only serve those drinks without any juice/other material that triggers false positives with the special straws. 

Hopefully, you can find special straws that look the same as the normal straws you give out so that an attacker wouldn't know whether the drink of a target has a normal or a special straw in it.

Then write on the drink menu that some false positives are expected and patrons who have their straw change color should simply come with the drink to the bar to get it exchanged for a new drink as safety precaution.

Whenever that happens you can do more expensive testing and if it triggers correctly investigate the video tapes.

Ban drinks on the floor and have gatekeepers guarding the exits to the bar. Train them to screen everyone leaving for symptoms of the date-rape drugs. (Are they fast-acting enough that this could be noticeable before they exit?) Announce and post signs so patrons expect this. Maybe train the bartenders too. Also don't give anyone more drinks if they're getting drunk. Maybe also screen everyone exiting the venue, if possible.

Have you considered banning the drinks altogether?

Ban drinks on the floor, and only serve them in sex-segregated areas. Have bouncers to enforce this. (Bouncers and bartenders could still be male, but in uniform.) A rapist would have to have a female accomplice, successfully pose as a female (difficult for most men), steal a uniform, or actually be a female (this has got to be rare), and then still somehow get their mark out of the female area against her will without the bouncers noticing. This wouldn't stop the boys from getting raped, but I assume this is less of a problem?

Your social circle is very different from any I've ever been in. Sorry to hear that.

I assume these drugs take effect gradually, and that the effects are dramatic enough that the victim is (eventually) aware of them.

If so, mightn't it be better to warn everyone about the problem (be on the lookout for drug effects!), and advise victims to inform others about the scumbags doing this? (Under the assumption that most of them time when people are drugged it'll be fairly obvious who did it.)

I'd think it would be better to drive the scumbags out of your community than to just find a way to prevent one attack vector. (Surely those who'd stoop to this will find other equally objectionable ways to achieve their ends.)