Ok i learnt this one the hard way, a bike i brought back in 98 always pulled one direction and if i lifted the front wheel it would headshake on landing, frame was twisted
easy way to tell is with a builders spirit level,
secure the bike and with the level on the side of the rear tire , get the bike upright (level will be about 45 deg from horizontal) , you may need a stand or jacks under the pegs, but make it secure as you will be moving the front tire
then when the rear tire is absolutely vertical , cross reference with the machined seat pads, they should be square or horizontal,
then level against the front tire , turn the bars to get the front vertical
NOW string line the front ....is the front pointing true or off to one side
If the swingarm is not true the rear and seat pad will not be 90 degrees from each other
and if the head stock is cambered the front wheel will not point forward correctly when upright
Watched the old school frame aligner guy do this to mine , and he did not have to tell me the bad news back then, i could see where it was going
Or you can take it to a laser aligner and get a printed sheet with your specs