If you think you might have a HG leak then you need to do a leakdown and compression test. A leak that allows coolant to go into the combustion chamber can be slow and hard to detect so you may not see any obvious signs.

Funny cause I finally tore my latest core appart and there was a blown HG story all played out for you. Basicly we tore the heads off and immediatly saw that the odd side had been replaced. On the short block you can see the stain from the coolant leaking into the chamber. When we took the bottom end appart we saw the full extent of the hidden damage. where the water leaked onto the piston it was cracked and broken etc. Obviously after they replaced the HG they didn't notice (I'm giving the benefit of the doubt there) the cracked piston. Thinking they had fixed the problem they drove it, and you can see that the rings and piston started to come appart. Leaving about a 15 thou gouge in the wall of the sleve. I'm sure the person driving the truck was driving it thinking, "doh I fixed it why won't it run right, stupid piece of junk 3.0 v6."

Two lessons here:

1. If you have a problem figure it out sooner rather then later.

2. Don't take the easy way out unless you like fixing things twice or losing you initial investment.