A couple of days ago my girlfriend bought a 94' Civic with about 180k miles on it. Before buying it we noticed a little bit of coolant in the engine compartment, which I dismissed as minor because it looked like the radiator was a little rusted and worn out. Since the car was being offered to us for only $750 and was running great as described by the family member getting rid of it, we bought it.

After buying the car my girlfriend noted that there were times when she would come to a stop after being on the freeway and smoke seemed to be coming out from underneath the hood on the passenger's side for moments at a time. The previous owner (family member), said they hadn't experienced anything like this, and since I hadn't seen it I assumed that she was just seeing smoke from burning oil or small amounts of coolant evaporating.

Yesterday we decided to try to get to the bottom of what was happening though, and so I followed her while she was driving. After about 10 minutes of driving on and then exiting the freeway plumes of steam/evaporated coolant started shooting out from under the hood and filling the inside of the civic. We pulled over to find that the radiator had sprung a big crack along the top that was about 3-4" long. The temp gauge was pinned at the top. When I checked the coolant it looked empty, and when I poured more in I could hear it inside boiling instantly. As the radiator filled up coolant simply started spilling out the crack, completely unable to retain it.

I'm led to believe that what's been happening is that the radiator was probably worn out to begin with (hence the initial bits of oil under the hood), but that as it was driven over the past couple of days it started developing this big crack (hence the momentary bursts of smoke at first and then the sudden huge plumes).

I know that the radiator isn't going to be too expensive or hard to replace, and so I'm sure that the most important question is whether or not the head gasket is blown. I was hoping to get some input on this, since the car doesn't really retain coolant anymore and so certain methods of diagnosing the head gasket won't work (i.e. I can't pressure test the radiator, I can't see if there's oil in the coolant, I can't run the car and see if there are bubbles coming up through the radiator). Would simply draining the oil and checking it for coolant be the best and most surefire way of diagnosing the headgasket? Do I need to run the car a little bit to let the oil froth up before doing this?

Hoping I can just replace the rad+hoses+stat and call it good but want to make sure that these aren't just going to be compromised again if the gasket is blown.

Thanks for your time and your help!