| Hi
I bought my 748 as a "damaged repairable", and its steering was tweaked. It was caused by the head stem being bent, which I gather is not uncommon. If your bike has only toppled over it is unlikely to be the cause of your problem, but if its had more impact it's a possible cause. Although it sounds bad, infact the cure was easy to do, with a replacement stem off ebay.
Checking the alignment is pretty straightforward. You need to project a line parallel with each side of the rear wheel, and check that with the steering straight ahead the front wheel is in the centre of the these lines, and also straight ahead.
You can project these lines by putting two long pieces of wood against each side of the rear tyre, so that they are parallel, and reach forward as far as the front wheel. Another way is to run a piece of string tied to an axle stand, or an oil can, at the front of the bike so it is straight, but touches the front and rear sidewalls of the rear tyre, runs round the back of the rear tyre and to the front of the bike to another axle stand, just touching the other side of the sidewalls of the rear tyre. It sounds complicated, but in fact takes longer to describe than it does to do!
I have tried to attach a sketch: | 
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