By removing the throttle body and looking inside the intake with a mirror and flashlight. I also suggest you read the plenum sticky on top of the forum.

 

Finally have an update..Checked the plugs looked OK but changed them anyway.
Checked the exhaust very weak pulled the cat it was plugged.Changed the cat checked the compression average 160 psi with 240k miles not bad..
Still need to check the plenum for leaks but the truck runs great now..Pulled a heavy trailer over the weekend no problem...
As for the Seafoam it works great.Had trouble a couple years ago with low compression on #6 cyl. with this same truck.Ran a can of Seafoam through the vacuum line on that side of the engine at around 1200 rpm and it clear right up. ( prob. what plugged the cat)
One more question though what kind of life span do injectors have.Should I check them or just replace them ?
Thanks for all your help...

 

Use a bottle of Techron in the gas to clean injectors. They should be fine. Cat clogs due to burning oil. Probably plenum gasket.

 

+1... Fix the plenum ASAP, you are going to ruin the new cat.

 

Ok....will do didn't realize the connection
Thanks...