A new method has been developed to detect and analyze molecular pi systems. The concept of bonding critical point is generalized to electronic pi systems, and it is shown how a pi bond can be characterized via the corresponding bond critical point (BCP) in planar molecules. In this context, charge density and its Laplacian at the BCP(pi) of a strongly delocalized pi system can be distinguished from that of a localized one. The presented formalism is applied to three types of nanoconductors as conjugated polyenes, which revealed the alternative pattern of the double bonds. Also, several cyclic conjugated molecules are considered to explore their pi electronic structure and aromaticity.