Booger1

The string is good for checking so you don't get side tracking,but that's it.

Short of having a granite plate or some known reference point like a jig of some kind,you can check for twist between the head tube and seat tube with a level by laying the frame on it's side,level the the seat tube,then check the head tube.Both should be the same.You can check for twist in the rear triangle with the same level.Set the bike upright/upsidedown,put your level across the chainstays as close to the seat tube as you can,level the bike,slide the level as far back as you can towards the dropouts,read level.Both should be the same.That will eliminate everything but the dropouts themselves.You would need a known straightedge or known reference that clamped into the dropouts in order to include them in your level measurements,or any measurements for that matter.

I don't know how bikes fit into trainers but is the front wheel on it?If the front wheel is off and clamped into something,that will come into play also.

The mushroom things will not check for frame twist,they will align the dropouts straight and parallel with each other,but without reference to anything else.They can be aligned perfectly and still not be parallel with the BB or front axle.The headtube could be twisted 90',those won't find it,as an extreme example.I think we all know that.

Just seems strange that that bike would track correctly when riding normally if there was any twist in anything.From what I'm reading,it seems like it's setup in the trainer somehow.