Head gasket, intake manifold gasket, or both? 95 Olds, 3100 motor

Being as there've been so many posts about intake manifold gaskets in here lately (great stuff) I figured it's time to ask this question...
Is it a head gasket or intake manifold gasket (or both?):
The woman has a 95 Olds Cutlass Supreme SL, 3100 motor, and it's eating coolant like crazy, roughly 1/2gal a week or so (this is just an estimate so please don't take it literally). It's definitely not leaking it, no puddles, etc. There is no coolant in the engine oil itself, she just had an oil change and there was no coolant in it. It had dex-cool but we flushed and filled with universal coolant (amazing universal coolant looks more green than anything, but that's besides the point...) The overfill tank has the tell-tale brown "crud" in it, as does the upper neck of the radiator. The old coolant looked like *mud* it was so disgusting.
Anyway, looking for advice, procedures, "make sure you do this and this and that" info, et al. Any and all help is appreciated, and if you need any more info I'll provide what I can, thanks in advance.
-GV
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GlassVial wrote:

Sometimes the head gasket leaks enough to allow * some * coolant in, which burns with the fuel, engine heats up, and seals tight
If you have a Radiator cap / Cooling system tester, the kind you pump up, test Cold, Warm, and Hot, leave cap off for test to avoid burns. Checking the plugs may find some plugs super clean [ steam cleaned ] others normal dirt.
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So basically you're saying do the coolant system pressure test (which is exactly the expense I'm trying to avoid here...it costs $75!) And I should pull the plugs, if some are clean and some are dirty, then it's head gaskets?
-GV
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look under airintake where hte intake meets the block and head also check forleaking heatercore
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There's no leaks of any kind that we've found, and the heater core is definitely not leaking.
-GV
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when you first start the car is the exhaustreally white in color or try to smell it it will smwll sweet if burning it. also does it stumble when its first started?
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