It may be obvious that a creature's holding a charge but not for the reasons one might expect
When a creature's holding a charge, Cast a Spell says
If you don't discharge the [touch] spell in the round when you cast the spell, you can hold the charge indefinitely. You can continue to make touch attacks round after round. If you touch anything or anyone while holding a charge, even unintentionally, the spell discharges. If you cast another spell, the touch spell dissipates. You can touch one friend as a standard action or up to six friends as a full-round action.
Emphasis and double emphasis mine. Onlookers can tell something's unusual about a creature holding a charge because that creature refuses to touch anything or anyone (except, y'know, enemies). Now, obviously, an extreme interpretation of this means pretty much never holding a charge: any held charge is expended almost instantly because folks can touch things with their mouths (so holding a charge means neither eating nor drinking), their butts (so no sitting, either), and their feet (so no walking), and do touch things with their whole bodies (so no wearing clothes). More likely, this should mean touch in the typical Pathfinder sense—that is, you must do the touching instead of something or someone else touching you—, but it seems, in this case, this kind of touching should be taken to some kind of extreme given how darn few ways there are for a creature to touch something unintentionally in the typical Pathfinder sense and the loud, pregnant warning in that description. (Note that identical vague language exists in D&D 3.5.)
So while the rules rarely address unintentionally touching anything or anyone, this rule exists, and the GM determines what this rule means and how far to take it. For example, the GM may rule that accidental discharge of a held charge can occur if the creature holding it doesn't have another feed him, clothe him, bathe him, open doors for him, and otherwise use its hands at all for him. (I'd argue things already in hand when a touch spell's cast won't set off a held charge but will if they're dropped and recovered, but ask your GM.)
In other words, if the drow ambassadress's unseen servant opens the door for her, have a peon shake her hand first.
Cast a Spell also says, "Touching an opponent with a touch spell is considered to be an armed attack and therefore does not provoke attacks of opportunity."
By one reading, then, the presence of even an undetectable held charge nonetheless, suddenly and inexplicably, sort of turns the crossbow-toting, robe-wearing beardy dude with the belt pouch full of poop and spiders into Bruce Lee. This is a less reliable means of detecting otherwise undetectable held charges because the holder needn't actually make attacks until he wants to, but when he does reach out his stinky, sticky hand to touch a foe and doing so doesn't provoke attacks of opportunity and there's no obvious reason he's not provoking attacks opportunity (e.g. he's apparently unarmed, he's assumed no hostile posture indicating the presence of the feat Improved Unarmed Strike if that's a thing in the GM's campaign), it's a pretty solid indicator of a held charge. That is, an attacker just can't fake having a held charge. (Can you imagine if folks could fake that, though? That'd be hilarious.)
"But spell effects are visible, right?"
Many spell effects that aren't called out as visible probably should be, but, technically, unless a spell says it has visible effects, it's only as visible as the GM says it is. Of course, some spells come out and say they're obvious, like the fireball spell's "searing explosion of flame that detonates with a low roar," and, more appropriately, like a chill touch spell's caster's "hand… glows with blue energy." But most spells—including touch spells—don't provide any guidance for their appearances. The spell shocking grasp, for instance, doesn't say it surrounds the caster's hands with comic book style lightning (although most assume it does because that's awesome), nor do subtler spells like touch of idiocy or imprisonment say anything of their visuals. (Yes, I've just called the spell imprisonment subtle—context is important here.)
Detecting a held charge using detect magic et al.
A touch spell that's cast and its charge held is a functioning spell. The rules for Magic on Duration has two relevant sections:
Touch Spells and Holding the Charge
In most cases, if you don't discharge a touch spell on the round you cast it, you can hold the charge (postpone the discharge of the spell) indefinitely. You can make touch attacks round after round until the spell is discharged. If you cast another spell, the touch spell dissipates.…
Occasionally a spells lasts for a set duration or until triggered or discharged.
This makes holding a touch spell's charge an aspect of a spell's duration. While the charge is held, it counts as spell in place and a detect magic spell's caster on the third round of the detect magic spell's use in the area of a held charge can make a Knowledge (arcana) skill check to determine the school of the spell effect (DC 15 + spell level) and, if successful, then make a Knowledge (arcana) skill check (DC = 20 + spell level) to identify the spell effect in place. (Note that the spells arcane sight et al. are just better for doing this, though.)
So to learn if she's holding the charge of an imprisonment spell, use the spell greater arcane sight to scan the drow ambassadress.