Li-Ion BMS - White Paper - Estimating the State Of Charge of Li-Ion batteries (liionbms.com)

With voltage and current information you can map (V,I) onto the VI characteristics to understand your SOC.

Even during GCD analysis you may gauge an idea about SoC of LIB.

There are several methods, such as Ah counting, OCV-based method, model-based method using equivalent circuit models and electrochemical models and data-driven methods with machine learning models.