While magic can still be a mysterious force in the current game, we can look at magic items a little less stingy than in the past. Potions are no longer the providence of spellcasters only 9th level and above, but can be made as low as 3rd level. No longer does one need to seek out exotic and bizarre components to make even a simple potion - a small investment of gp and xp can produce a wide variety of magical brews, elixirs and potent potables.
On the receiving end, characters can even attempt to identify potions without long hours of spellcasting or the expenditure of gold via identify to reveal a potion's properties - a successful Spellcraft check can quickly tell you what sort of potion it is that you're toting around.
Yet, still some of the "old ways" abound. When potions are found, most DMs still do not identify the potion straight away, and it may be several adventures before one either safely identifies the brew or becomes desperate enough to attempt to imbibe the magical liquid. Other DMs don't bother with the mystery of discovery, and dispense the potion's name immediately on acquisition.
However, neither extreme case need be the way to handle potions in the game. Those wishing to retain the mystery of potions, yet allow for methods to identify (or just spruce up their descriptions) may find interest in the advice below.
Spellcasters who are creating their own potions need quick and easy ways to keep track of their inventory. This is the case with any item a character has whose appearance does not automatically relay its function to the user. We see this every day in our own world - from people who label folders in file cabinets to the label that appears on a can of string beans in a grocery store. Imagine the chaos, for example, if cans of vegetables were sold without those labels.
The easiest way for spellcaster to keep track of a potion is to physically label it with its contents. This method makes the most sense for any potion destined for another person (say, perhaps potions of cure light wounds given out at a temple, or a town's potion-brewer, who sells his wares on a regular basis).
Often, the formulas for a potion will have been something the spellcaster has learned or gleaned from another, and thus potions created from the same formula will often have the same appearance, taste and behavior. (again, perhaps everyone knows that luminous blue potions are potions of cure light wounds created by the temples to Pelor in Greyhawk). At the same time, it is possible that due the variance in materials available to a potion maker, that two potions may have a slight or even a dramatic difference in their appearance, taste or behavior.
Third, the container that is used can often be used to identify the potion within - or perhaps, who made it. (using the same example, perhaps it is traditional to store potions of cure light wounds in a lead crystal container with a stopper made from a femur.)
While it would be possible to go into all sorts of complex formulas, I suggest a flat 35% chance that any given potion is clearly marked in some ways that its contents are instantly identifiable.
Second, while the DMG encourages DMs to allow a player to identify a potion by taste, there's no real mechanics; identifying a potion encountered before is either automatic or up to the memory of the player. I believe that if a character is attempting to identify a potion he has previously encountered it would be appropriate to grant the character a +10 bonus to the Spellcraft check (and, in this instance, to allow the Spellcraft check to be used untrained).
Lastly, to help ease the DM in describing potions, it sometimes helps to define the colors, taste and appearance based on schools of magic. Each can be further tuned by the spell held within the potion, as the DM decides. The DM is, of course, free to change the colors, tastes and appearances about as he chooses, but I highly urge that spells within the same school have a "theme" both to help the DM remember what an unidentified potion might be and to possibly assist players in identifying the brew. Here is my suggestions:
Abjuration = Silver or Grey; Bitter or Bland; Viscous or Oily
Conjuration = Tan or Brown; Meaty or Earthy; Thick or Gelatinous
Divination = Blue or Clear; Almonds or Honey; Frothing or Foaming
Enchantment = Pink or Purple; Smooth or Tasteless; Watery
Evocation = Red or Orange; Heady or Spicy; Swirling or Bubbly
Illusion = White or Multi-hued; Sweet or Reminiscent of Another Food; Glittering or Flashes of Images Within
Necromancy = Bile Green or Black;Brackish or Rotten; Laced with unidentifiable Bits or Contains Some Bit of Anatomy*
Transmutation = Gold or Yellow; Tangy or Fruity; Sticky or Paste-like
* For example, an eye floating in the bottle
