The increasing integration of renewable resources via power electronic inverters is shifting a modern power system toward a 100% inverter-based power system (IBPS). To maintain the stable operation of a 100% IBPS, it is important to identify the small-signal stability issues resulting from the interaction between the power network and inverter-based apparatuses. While grid strength assessment is a useful tool for quickly identifying the small-signal stability issues, the existing methods are not applicable to the 100% IBPS dominated by grid-following (GFL) and grid-forming (GFM) inverters. To fill this gap, the paper proposes a method for assessing small-signal grid strength of the 100% IBPS in order to quickly identify the small-signal stability issues from the perspective of grid strength. First, we formulate a multi-inverter system modeling for the small-signal stability analysis of the 100% IBPS. Then, based on the analysis results, an index is proposed for quantifying grid strength, and its threshold is also analytically defined to characterize the system stability boundary. Also, an analytical expression is derived to determine the threshold and analyze the impacts of GFL and GFM inverters on the stability boundary. With the defined index and its threshold, our method is proposed and then validated on a modified IEEE 39-bus system.