1997 Mitsubishi Eclipse need to know if my head gasket is b

I bought this car off craigslist for 2000 grand from a guy who just moved here, it drove fine with a few minor repairs, well the day I bought it I drove it across town to a friends and we noticed it started over heating so we turned it off and poped the hood and let it cool off for about 5 hours. Well I called the guy and he said coolant or something had a air pocket just needs to be drained. Well my buddy went to start it 5 hours later and it ran great they left it running for 30 minutes then drove it around for 30 minutes no problem the line stayed in the center perfect! Well I ran it by a shop to get it looked over and they were very rude, and said they didn't look at it at close time but then called me back and said head gasket bad that will be 65 dollars for the check up! So I was in shock but still in the past day and all never overheats no white smoke from the exhaust or anything, but on the other hand my friend said my oil looked creamy but the oil was changed right before I bought the car and also in the overflow tank there looks to be grime and dirt all in it which could be dried up oil or dirt but when I look in to see the water it's clear I think the cap wasn't put back on it and over time with off roading caused that build up, but the final question is how do I know for sure my head gasket is bad and it's not something way more simple, and what are good ways to check it, cuz it drives great no problem. Thanks and will look forward to your help-drew