genrally a PCI-E slot will have a sort of...Release mechanisum on the right hand side of the slot...either a pull/push leaver or a pull/push pin sort of thing.
Or like mdd1963 said, try the mobos website or google the mobo + specs
E.G. Asus p5KPL/1600 Spec
usually gets you some resualts for about 1 minute of work =D