The applications of ultrashort laser pulses (< 1 ps duration) for micromachining have continuously expanded during the past decade. In particular, femtosecond (fs) laser sources represent unique tools for microprocessing solid dielectrics such as glasses, crystals, or bio-compatible materials. The chain of physical events leading to laser-induced material modification starts with the formation of an electron-hole plasma which essentially proceeds through two distinct channels: strong-field ionization (SFI) and electron-electron impact ionization (EEII). In solids, these mechanisms take place intrapulse, i.e. when the electric field from the laser pulse is still present in the interaction region. Because of the short timescales involved and because of their concomitant character, determining the relative importance of the ionization channels at play is very challenging. In this article, we describe a strategy to determine the relative importance of SFI versus collisional ionization in the plasma formation when irradiating solid dielectrics with ultrashort pulses.