If you don't have a line of sight direction, then the best way is with a signal strength meter. Contact a local dealer, they should have one.
They may even be able to provide you with a tower location from their network guys (I have an internal online map for our network in Canada) and give you a pinpoint to aim at, which with a few degrees of variability will be fine.
Without these tools, you'll need a bit of tinkering.
Are you hooking up to a modem or a cell booster?
Easier to position a modem with the -dB readings.