We can just eyeball it, basically. For most applications, it simply isn't important if the laser is exactly perpendicular or a degree or two off in either direction. If the angles in each direction differ slightly, we can just average them.
If you do actually need to line it up precisely, you can measure the two diffraction angles in each direction,` theta_R ` on the right and `theta_L` on the left. You can then slowly adjust the positioning of the grating until these two angles are exactly equal to within the available precision. The precision of alignment depends upon the precision of your angle measurements and the precision with which you can move the grating.
Like I said, though, for most practical purposes, you can just line it up so it looks close enough. Sometimes exact science isn't an exact science.