In most applications, an interference filter should be placed with the most reflective, metallic looking surface towards the light source. The second surface of the filter is distinguishable by its more colored or opaque appearance.  Many companies (Semrock included) have an arrow on the edge of the filter.  The arrow indicates the direction that light should travel for Semrock and many other companies.  If it's a different company, you should probably double check what the arrow means.