If the surface that you are drilling is large enough, try the following:

Find a flat plate of Al sheet metal (thickness doesn't matter) and drill a slightly oversized hole in it.

Put it on the surface you want to drill with the hole aligned with the hole you want to drill.

Align the drill until the bit appears to be straight with the reflection of the bit. Do this from two directions. When the reflection appears straight, you are perpendicular to the surface.