A method for determining a presence of at least one guest structure at a host structure. The method comprises a light-sensitive system receiving light from the host structure. The host structure hosts one or more optically active entities at at least one part of the host structure. The at least one part does not host the at least one guest structure. Furthermore, the optically active entities cause light emission from the at least one part. The method also comprises the light-sensitive system outputting a signal based on the received light a step of determining a light value based on the output signal. The light value indicates an amount of light from the host structure incident on the light-sensitive system. The method also comprises determining on the basis of the light value at least one of a quantity and a position of the at least one guest structure.