Well, I didn't get out on the bike to-day, but I did get to spend some time in the shed assembling my new bike....and on the way I took some photos of "how to check your frame is straight" ...using that most sophisticated of tools, a piece of string. I actually used baler twine, as its free and fairly easy to photo in red.
Pass your bit of string through the valve hole of the front wheel, put the valve hole at the top, tie the bottom end of the string to the rim opposite the valve, and the top end to the handlebar stem, with the string tight.
With the steering tied "dead ahead" and squinting from dead in front, the string line must pass dead centre over the head tube, and dead centre of the hub, with no kink at the valve hole.
With the bike upside-down, pass your bit of string through the front wheel valve hole, and tie a washer or anything else that won't go through the valve hole on the hub end of the string. Hold the string above and behind the back wheel, pull it tight enough to centre the steering. (My steering is centred...it must be distorted by the camera lens)
Squinting along the bit of string, with the steering held dead ahead, the front wheel, down tube, BB and back wheel must all be dead in line.
The back wheel should ideally be dead in line with the seat tube too......I wouldn't worry about a small error here in an old frame if it was OK everywhere else.