Cement is hydraulic: it starts reacting with any moisture it meets, including damp air. A bag that has taken moisture forms lumps and loses strength, and hardened lumps should never go into the mix.
Store bags on raised timber pallets or planks, at least 150 mm off the ground and away from walls so air can circulate. Cover the stack with a waterproof sheet, not just a dust cover, and keep it off the tops of the bags where condensation collects.
Do not stack more than about ten bags high; taller stacks crush the bottom bags and can topple. Mark deliveries with the date and use the oldest stock first.
Before use, squeeze a few bags. They should feel like soft powder. If a bag has hard lumps that do not crumble under light pressure, set it aside for non-critical use or reject it. Order quantities you can use within a few weeks of delivery rather than warehousing cement on site.