Noticing an animated object

[Hogeyhead]

I don`t know if this should go under advice or rules, but I`m putting it here for now.

I want some chairs in a room to secretly be animated objects that attack the pc`s at an in oportune time, what I wanted to know mechanically how should I handle noticing the construcs? They aren;t really hiding so stealth doesn`t make sense. I`m thinking sense motive or perception but how do I calculate the dc`s?

Thaanks in advance

[Lobo Apache]

I would say that someone uses detect magic it would shine like a Christmas tree, but your best bet without this would be perceptions and starting with pretty hard (level based maybe 10+ caster level of the animator) then making it easier by making the items move around or something to that extent.