1999 Subaru Forester

Engine Cooling problem
1999 Subaru Forester 4 cyl All Wheel Drive Manual 95,000 miles

I bought this car a year ago and have drivin it accross the state to/from school 6 or 7 times now (500-600 miles one way) and the the last 2 trips I went through almost my entire coolant reserve each way. I had a pressure test done at a couple shops and none of my hoses are leaking, there is no oil in the reserve or radiator, and there doesn't appear to be any water in the oil. I don't have any issues with over heating, even when I drive the 500miles straight, and I dont seem to be burning/leaking it when I'm just driving around town; I filled up the reserve when I got back home and all my trips have been less than 20 mins, and I havent had to add coolant the past few months. I park on gravel so it's hard to see if it's leaking on the ground, I did put a piece of cardboard on the ground for a week and only oil drips came up (I expected that). I do get white smoke/steam from the exhaust when I'm accelerating from a stop. I dont really get any when it's idling and not a whole lot while driving, but after sitting at a light and it turns green, I get enough that I can see quite a lot out of my rear view mirror.

The shops I went and explained this to said to watch the coolant and temperature since it's not the hoses, and it's probably a head gasket, but since it's still running great and i'm not having overheating issues I can wait. Should I wait? Does it sound like a head gasket? How can I find out for sure what it is without asking for a head gasket job? The shops I went to seemed nonchalant about it and not very helpful.
April 4, 2010.

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