The easiest way is looking at the engine size. The neos have way bigger diameter engines -the CFM leap 1a or the Pratt and Whitney pw1100g- look way bigger than the ceos engines. The ceos have the smaller CFM 56-5a or IAE v2500 engines. Also a third lesser known and rare pw 6000 engines on some a318s (iykyk). Looking at the fan (big spinning propeller bit at front), the neos have bigger, wavy looking black/grey blades with silver leading edges (carbon composite construction) vs the ceos which have very metalic (titanium alloy) straight blades and more blades in their fan. The neos have a smoother flowing pylon mounting the larger engines to the wings, while ceos have kinda awkward metalic pylons that look straighter and more geometric. The neos also have strakes (fins) on both sides of each engines, while the ceos dont. Neos: Cfm leap: less blades, black ring around fan, white spiral pattern painted on spinner (cone in front/center of engine). Pw1100g: green ring around fan, whale like noise can be heard at low power settings. Single white teardrop/notch painted on spinner Ceos: Iae v2500: long engine shape, no metalic cone can be seen in rear, as there is an exhaust mixer inside the cowling.metalic ring can be seen on the exterior of nacelle. Small probe can be seen on inside top of intake. Sometimes seen with yellow spiral painted on spinner. Cfmi 56: exhaust cone visible, shorter engine nacelle, some have cheverons on the tips of the exhaust cone. Typically more rounded spinner design.

Also another fun fact, the leap 1a's take longer in their starting procedure as the engine management system has them "motor" or spin longer with the starter before introducing fuel due to the engine re aligning and ballencing the main shaft which can sag after shutdown of hot engines.

Happy spotting!