How do I tell if my head Gasket(s) are blown? What else can I look for?

What is your question?

Just had my Serpentine belt replaced drove about 3 miles check engine light came on? So I took home parked then drove it 18 miles and engine lost power and a big cloud of smoke come out back of the truck & it just died! I pulled over and there was a caramel colored fluid coming out! Firemen just behind me pulled over, they said I had a blown a head gasket? Which gasket set should I purchase & do I need to sets of head bolts as my Blazer is a 4X4? Before this all happened I had seen what looked like petroleum jelly at the top of my oil cap & where I put oil in at?? I have replaced the radiator ($220), the over-flow tank ($30), the metal elbow looking thing at top of engine & a brake sensor ($1164.40). And the "S" belt mentioned above ($385). Quick ?? ~ Shouldn't the owner/operator (mechanic) of the shop that replaced my "S" belt have at least checked my oil, water, etc., before letting me drive away from his shop?? Thank you in advance for your attention into this matter.

How long has your 1997 Chevrolet Blazer had this problem?

6+ months