Optimally coupling light in an integrated Photonic crystal (PhC) cavity is challenging, but crucial for improving their sensing properties. Here we experimentally investigate the impact of side coupling and in-line coupling on the transmission properties of integrated silicon PhC based air-slot cavities by probing the near field of the cavity mode with a nano fiber tip. These cavities were fabricated with standard deep UV lithography. Positioning this nano-tip near and inside 130 nm wide PhC slot cavity modifies the dielectric map of the cavity which perturbs the intensity scattered from the cavity surface. We show that the mapping of the nano tip induced intensity variations provides some insight about the nature of the confinement of electric field of the various modes of slot cavities. Such intensity maps carry moreover information about the cavity light coupling, which is useful for maximizing the intensity of PhC slot cavity modes.