Well, straight down anyway. I'm sure many of you have seen this idea multiple places (including commercial offerings like DeWalt) but, for those who might not have. These bullseye levels are available inexpensively. You may even have one laying around in an old tool box or drawer. I was transferring this one from an old 9.6v drill that had lived a good life and moved on to the clearing at the end of the path.
Clamp a straight rod in your vice. I used a block with a hole drilled through and then split it on the bandsaw to give me a more reliable grip. Use whatever method you have handy to assure it is perpendicular to mother earth (standing straight in all directions, that is).
Chuck your drill on and re-confirm alignment. Draw a ring of silicone rubber glue or the thick goop of your choice on the butt of the drill motor. Press the bullseye level in and align it for dead center.
Come back tomorrow and start using your drill motor ;-)