go to NAPA and get a Block Tester kit about 50.00.
This will test the coolant for hydrocarbons, i would flush it once, then refill with water run the truck around the block then test the coolant make sure to leave space in the reservoir.
The block tester will sample the coolant vapors for hydrocarbons .
If you your test is positive then a headgasket is bad or you have a cracked head,

Also this can also be
verified by looking into the reservoir with the cap off and at temperature, use coolant for this test so it wont boil over ( do this one before you flush) You will looking for bubbles coming out of the coolant tank , this indicates a failed headgasket or cracked head or cylinder

 

Note also that if you're leaking out your bleed screw without having purposely loosened it, then you probably need to replace it...