Checking 'plumb' when mounting bike on trainer From my former questions, y'all might be aware that I prefer to ride indoors  - for various reasons 


no hordes of people of median IQ in 1500kg death machines waiting to smoosh me;  
no need to wear lycra or Kardashian-shoes or helmets; 
no 'free travel' by not pedalling on downhills.


My Wahoo KickR is currently at a mate's place, so I've had to go back to an old "dumb" trainer; that's fine, because I've got a built-in power meter on my bike now, so I can still calibrate my workouts to power and HR zones.

One thing I've noticed on the dumb trainer is that I am very sensitive if the bike's not "plumb" (worse, if it 'shifts' during a session). What I mean by that is that it's very slightly tilted from vertical along its long axis, or if, having felt relatively 'plumb' it moves out of plumb during the ride (oddly, it tends to tilt left more often than chance would indicate). 

I can compensate by turning the front wheel in the opposite direction to the 'tilt', but that puts significant strain on shoulders and arms if I'm training remotely hard.

It's not 'Princess and the Pea'-levels of sensitivity, but it's noticeable; by the 50 minute mark of a ride, if the bike's 'leaning' I get preoccupied by it, which screws with concentration and nek minnit my scranus is killing me and it's easier to just get off the bike.

Since the top tube on the bike is wider than the down tube (see? I know all the words now), to measure 'plumb' would seem to require taking a plumb-line down both sides of the top tube and ensuring that the gap is even on both sides. That's a gigantic pfaff, and I'm sick of it already and I've only one it twice.

I had thought perhaps that I could try to run a single line from a loose loop attached to the top tube (hoping that it intersects the down tube centrally), but that doesn't seem to work.

Has anybody ever bothered to actually do this, or do people not care much? 

If I was 70kg or thereabouts I would not be that concerned if the bike felt like it was listing like a drunken galleon; it would be unlikely that the entire thing would come adrift. 

As it is, I'm ~104kg - if I can notice that the bike's not 'vertical', I'm betting that there will come a day when gravity will notice as well (and given Sod's Law, it'll probably happen when I'm sprinting).
There are several options, ranging from low tech kludges to actually solving the problem. So, starting from low end:


Loop a string around your top tube and tie so that loop is loose and the string can move. Tie the weight to the other end of the string. Now you have a plumb that hangs at the center of the tube.
Get a spirit level. If the solution needs to be free and you have a smart phone, there are also apps for it.
Higher tech variation: Get a laser spirit level and fix it to the frame so that the line shows at down tube. Now you can check the bike position by just looking down.
Actually solving the problem, option 1: Forget the trainer and use rollers instead. You will now maintain the bike upright automatically.
Actually solving the problem, option 2: It sounds like you thorougly dislike cycling. Try some other kind of exercise.