|This is from Moldvay, not Holmes. But you get the idea.|
Some method of detecting the effects of the potion must be found. If the characters lack a detect magic spell, they may dare a tiny sip to see what the result may be.
A spell to determine if there has been some enchantment laid on a person, place or thing. It has a limited range and short duration. It is useful, for example, to discover if some item is magical, a door has been "held" or "wizard-locked," etc.
That's how I see it: A character can use Detect Magic to figure out exactly what a potion will do. Would you allow this in the campaign you run? Would you let players use this spell to identify other magic items as well? I think I might.