How to check a bare frame for straightness

Before I send this frame to the powder coaters, I would like to check for straightness.
How would you go about verifying this?

Bob

 

String (or chalk) and a measuring tape.
Set up parallel lines running from the steering neck to the rear of the frame, set at the frame width, and a third running from the steering neck to the centerline of the rear of the frame.
You can now eyeball to see if there is any bend, and/or measure.

To assess twist you'll need a length of tubing, pipe, or rod that fits inside of the steering neck (best), or can be clamped to the outside of the neck (fiddly, but works) and is both square at the ends and true along its length. measure the angle where it hits the floor relative to how the frame wants to sit onthe floor. You can also measure for the correct fork angle at that time.

Ideally you'd actually build a fixture for the steering neck, but the above method will catch anything that would make the bike unsafe to ride.

 

I did that with string lines to set the toe in on my car front wheels. I am a Carpenter by trade. Tyres wearing even now .