You can check for warping with a straight edge and feeler gauges, but if you want crack/pressure testing you're going to need to build a jig of sorts - flat plate cut to fit and bolt to the flanges, with either a gasket or RTV sealant, drill a hole somewhere and add a Schrader tire valve (get one of the flanged screw-in ones for alloy rims), pressurize the manifold and immerse it in a tank full of water (try either a radiator shop or a tire repair place).
Apart from possible warping of the flanges, I don't see much else as being an issue - it's not like the intake is a highly stressed part.