Hmm..while I'm not sure if you can buy anything to test a horse for drugs. I'm sure you can bring a vet or take hair samples of the horse and give them to a vet to test for any drugs used.
Some signs to look for in the horse...
-consistent cocked leg.
-dull, lifeless expression.
-bottom lips always hangs, even if you try to make him jog/lope/etc.
-head hung low consistently
-not amused by anything
However, these are also signs of a very, very well finished horse...I have a friend who had a late cut gelding. He's not drugged yet he does everything I mentioned on the list, but he does spook and will give a lively expression here and there.
If the horse has all of theses signs, and possibly more suggested by other people then you may want to look at the owner/trainer of the horse.
-very nervous, reluctant
-reluctant to let you use your own vet and instead offer a vet that they chose.
-not willing to let you go into the barn, namely feed room.
And other that I'm sure you will be able to spot...
I've never dealt with this situation and the things I have told you I was told from others who have faced this situation. Best thing to do is have someone come along, who maybe has dealt with this situation before, and have them give a second opinion.
Another method that I was told is to touch the horses around the genitals. Most will flinch or tense.
Like TheAQHAGirl said, some horses can show signs of being drugged when they aren't, like my gelding.
It is very difficult. I feel that my first mare was drugged, as she was docile and simply wonderful for our test ride, but was pure racehorse once I'd bought her. However, the only signs she showed were those that can easily be attributed to a laid back or lazy horse.
Your best bet is to visit more than once, and if you can, show up 'unexpected'. If you're like me, you'll feel a little uncomfortable doing that though, so even arriving early can help or simply saying that you'll be there sometime between A and B.
Im including blood tests when I get a vet check but only after I have tested the horses vital signs numerous times. Unfortunately the horse im checking is 4 hours away so me and dad r staying the night in the country and if we like the horse on the first day than the second day we will get a vet check and float horse down to agistment.
I think very few horses are actually drugged these days. If you were to ride the horse I imagine they'd be quite unresponsive so its not in the sellers best interest. I've been involved in horses for 10+ years and never in that time have I seen a drugged horse for sale, or known anyone else who has. It's all just stories. I'd be aware of it, but not worried.
Many people think their horse has been drugged because when the horse comes to their place, the horse's personality changes. I think this is pretty common, because when a horse is at a new place with new people they start pushing boundaries. Different environment and dynamics.
There is only one way to know if a horse has been drugged to make it quieter for riding and that is a blood test.
I agree with Saskia in that a horse's personality might well change in a new environment but I have known horses to be drugged both with light sedatives or with pain killers to make it sound.
I would go try and find a horse. Then arrange to have it vetted and request the vet to draw blood at that time. One phial is given to the owner and you take the other, Then if anything goes wrong the blood can be tested if necessary.
Another thing is that when the horse is vetted you should be present. I have known people go try a horse and it was what they wanted. The vet went along several days later saw it ridden by a good rider and noticed that the horse was fairly 'hot' but it is not for him to say so in the vetting. Bloods taken but, nothing to be found as the sedative would have long worn off. Had they been present then they would have seen it was a 'different' horse.