Fluoridate the water supply:
A funny side effect of Fluor in combination with Amortentia is that the person falls in love with greens instead of people:
They start eating large amount of salad,
Go lay in the grass all day long (even when it's raining)
Wear green clothes only.
Start keeping green frogs as pets
Look what it did for the city of Ank-...
If the problem is so profound your entire court could fall to these hags drugging the lords and ladies, actions will be taken.
Coordinate a campaign with the Grand Maestre to vaccinate your population against the most common version. We still can get sick from a special strain of a disease, but 99% will be harmless.
A Potion Master working in ...
If it's magic, why not magic to counter it or render it inert? You could get various different possibilities if the magic is a charm someone carries, a one-time potion inoculation, a potion that has to be drunk or a spell that has to be cast at regular intervals, and so on. There'd probably be ways around all of them (a potion to negate the potion that ...
If you can't detect the chemical, perhaps you can detect the effects.
Since magic exists it could be possible to detect if someone is bewitched, even if you can't detect the magic (potion) that caused it.
Mandatory testing during marriage ceremonies would eliminate those marrying into wealth through deception.
And making it possible for a concerned friend/...
Make everyone use it
If its cheap harmless and makes you feel good why not consume A LOT of it? Think refined sugar. People no longer let random hormonal states control their lives. Why would they if you can feel love for partner of your choosing(Choosing it self will probably change in some way). When everyone is taking love potions it will stand out big ...
A simple and presumably in-genre solution would be enchanted rings which detect the drug in drinks.
cf items like
... and the debate as to whether this is correct solution.
Love potion is basically a euphemism for date rape drug. Most people IRL don't use date rape drugs. Why would your world be any different?
How to regulate drugs?
You take a piece of paper, write "list of forbidden drugs" on top, then put "love potion" under it, then pass that paper to your legislature. Now, that may be a gross oversimplification, but that'...
Enforce arranged marriages, putting love out of the picture. Let the arrangement be decided by a third party on the basis of convenience or fate, maybe through a lottery.
Whoever opposes this scheme will be punished.
In this way whoever gives and receives the potion will stand out. Then it will become a matter of lawyers to say who is guilty of what.
The potion might not be detectable, but it's workings could be.
The nature of love would be much easier to investigate with a potion to create it. This becomes especially true with deviants (not negatively meant) who will take a lovepotion before sex (or even just a temporary loving relationship) for the thrill and repeat it the next day/week/month with ...
Even thought the ingredients are common, that doesn't mean you have to stop the sale, only control it.
Think about what the US government has done to try to reduce Methamphetamine production in the US. A quick and dirty way to produce meth is to use psuedoephedrine at one point in the process. Pseudoephedrine is a common and effective decongestant and was ...
Bottom line is that you simply can't prevent it. Harsh penalties don't work: see e.g. Prohibition, the "War on Drugs", attempts to censor pornography or "subversive" writings ("samizdat" in the USSR), and so on. So your society has to learn how to deal with it, perhaps by making it socially unacceptable, as with tobacco in recent decades, or by inventing ...
Even if detection is absolutely impossible (which is up to you, but I imagine forensic alchemists would toil - and trouble - deep into the night to find a brew that revealed love potion's effect or presence), there's an easy parallel in real life - date rape drugs.
Often no traces are left by the time the victim is in a position to have ...
If you suspect that someone you know might be a victim of a love potion, the law allows you to call for a test to be done on them to check for the presence of the relevant chemical signatures in their bloodstream.
Ordinarily, doing a blood test on someone without their consent might be an invasion of their privacy, but the law allows it in ...