First, make the best call you can with the information available. If that is foul, call foul and live with it.
If you have a partner, and he is in a position where he could offer some help, the suggestion to rule it fair and then ask for help is a good one, since you can always go back to foul after the fact, but you can't go back to fair once you kill the play. Sure, you can attempt to place runners, but that is always messy.
Or, you could use the "who's fault is it, anyway" rule... if the base coach or a runner is blocking your view, it is foul. If the fielder is blocking your view, it is fair!
