I use a metal yard (or meter if you prefer) stick as an alignment guide. Place the yard stick on edge over the stem's or steerer's center bolt and have it extend over the exact center of the handlebar clamp (the center of the handlebar clamp can often be located accurately by using the logo on the face plate as a guide) and out over the front tire. Look at the yard stick from above, If the stem is centered, the yard stick will exactly bisect the front tire.