Unless it is in an effects loop of some sort, or there is some kind of clean blend then it would almost certainly be in series. Boost into drive would be similar to using higher output pickups in that it hits the drive pedal with more signal, this tends to give more overdrive/distortion/fuzz and depending on the circuit it will probably sound more compressed. If you ran the drive into the booster, you just get a louder overdrive pedal, similar to tweaking the master volume on your amp. This of course is assuming a pretty uncoloured boost, otherwise you get all sorts of interesting and useful tonal fun going on (Mid boosts, and various treble boosters spring to mind) Ultimately there is no right or wrong order though , and each has a valid use depending on what suits your particular needs.
If you are unsure or want to experiment a little bit then it's easy enough to fit a toggle switch to change the order of the drive and boost.
If you haven't got one already I'd suggest that you get a breadboard and try a few different boosts to see which suits your overdrive the best before boxing up, as some have more pots than others, and you might even find a favourite for in front, anddifferent one for after the drive. Switching for that would be a little bit more complicated but certainly do-able as long as you use have reasonable skills and a suitabley sized enclosure. Check out the offboard wiring posts to get an idea.