Its fairly common to have the intake manifold gasket leak, its really uncommon to have a manifold crack, not sure if anyone here ever had one crack. However, a gsaket leak wouldn't necessarily sound like it was coming from the input side of the manifold, but there's not a lot of distance between the two, and I suck at sound isolation of problems.
I only found one thread about an intake manifold crack, viewtopic.php?t=29466
where sparrow's mechanic said the intake manifold was disintegrating, but it turned out to be nothing actually wrong with the car
Not sure how you'd test. I know some shops can check for vacuum leaks by pumping in some sort of fluorescent smoke, leaks show up with a UV light, but I've never done it or had it done.
Depending on where and how significant a crack is, a surface patch might work, but if its in a place engine vibration could somehow flew it, it might not hold.
Are you having any performance issues or perhaps a check engine light with a code that indicates a lean condition?