How to know if a C15 engine is good or bad

Get the codes read. Send in a sample.
If you arent confident enough get a mechanic to look at it. You could even get a shop to do a safety on it just to give it a once over. Cheaper then getting a trashed truck.

 

I don't know about that, I know a guy whose got them and loves them to death, but he's pushing 700 horse or something like that out of that thing, apparently you can hook it to 100k gross and drag it around like nothing.

 

You usually won’t get anything out of the blowby tube until the motor has been pushed a little and up to operating temperature. But don’t be alarmed. Your gonna see a little smoke mist coming out of any blowby tube.