Quantum interference (QI) effects in molecular devices have drawn increasing attention over the past years due to their unique features observed in the conductance spectrum. For further development of single molecular devices exploiting QI effects, it is of great theoretical and practical interest to provide simple rules controlling the emergence of QI effects like anti-resonances or Fano line shapes. In this study, we propose a minimal toy model to predict the appearance of quantum interference, Fano resonances or anti-resonances, and its position in the conductance spectrum of a generic molecular junction with a side group (T-junction).