you'll need a long string
make sure front wheel is straight.
-- you can do this taping hte string to the rear wheel, then pulling the string taught to hte front wheel. measure the gap at the rear of the front tire. when the gap is even on both sides, it's a straight front tire.
now, in a similar process, you can make sure the rear is aligned.
-- tape the string to the straight front wheel. measure the gap between the front of the rear tire on each side. once those two gaps are even, your rear tire is straight.
sounds complicated, but in practice it's easy.
sorry if it's a confusing write up