You can have your cooling system checked with a pressure tester through the radiator cap. Most radiator shops can do this.

A rusted out freeze plug is more likely than a cracked block, but only a pressure test could tell for sure.

In the past the sure sign of a blown head gasket was LOTS of white smoke pouring from the tailpipe (coolient being sucked into the cylinder and coming out as steam), but now it is common for cylinder pressure to leak into the cooling system and all you get is an increase in cooling system pressure.

It could also be that the bottom of your radiator needs to be cleaned out. Is the bottom hose of you radiator sucking down when the engine gets hot? If so your radiator is full of sludge that has settled into the bottom.