My gut feeling is it should be negative pressure. If they designed it for positive pressure the chance of it getting plugged with contaminants your trying to exhaust would be too high. Even if they have it after the fan, if it is pointing down stream, it'll measure negative pressure, which is what I imagine it does. Either way it won't hurt anything if you screw it up. It'll either work or it won't
If it doesn't, just switch it.