Surprise when the bad guys are faking it

[bmcdaniel]

I'd give them a DC20-25 spot check to notice that the skeletons have glowing red eye sockets (per the description in the MM). If they fail, skeletons get a suprise round.

 

[ThirdWizard]

Hm. Maybe handle it like a mimic; the situation is quite similar.

I was thinking gargoyle. Gargoyles receive a +8 racial bonus to Hide checks when concealed against a background of worked stone. Maybe give zombies a +8 racial bonus to Hide when pretending to be a dead body. Gargoyles aren't "hiding" exactly, they're in plain sight, so Hide can be used in this way by the rules.

 

[Ridley's Cohort]

As I said I agree - in this case the check's is so hard you've got to make a spot vs. DC 20 and vs. Hide. But I just can't get around the fact there's nothing to indicate there's a threat until the attack is made. Perhaps if you wanted to insist on a skill check a heal or knowledge [religion] against DC 20 would notice the signs of unnatural magic on the bones (though would this mean you knew they were undead?).

As for assigning DC, the default DC for something that is not impossible but implausibly difficult for a non-hero is DC 20. That is the DC for noticing the presence of an invisible creature which is a fair yardstick for 'nothing to indicate there's a threat' in my book.

I suggest a DC 20 and let the PC use the best skill that seems reasonable to you (Spot, Heal, or Knowledge Religion).

The only likely circumstance modifier is extremely bad lighting. Since the skeletions will probably not attack until within 30' of most of the party, that is unlikely to apply.