With open curves, there’s no way to tell. The offset direction will depend on the curve direction from start to end. If you reverse the curve direction, you reverse the ‘natural’ offset side (with a positive distance input).
If the curves are somewhat organized it may be possible to find a single point that indicates the offset direction for all curves (like the native Rhino command), in GH you would then need offset both directions, check the distance from the point to some point on each of the offset curves and choose the shortest distance. The choice of the offset direction point and the point on the curve to measure will be critical.