General Rule - Most 25.4mm clamp bars will have a 22.2mm grip area and will not accept bar-end shifters/levers. 26.0mm clamp bars have a 23.8mm grip area and do take bar-end shifters/levers.
22.2mm grip areas are designed for MTB shifters and brakes. 23.8mm diameter work with road bike brake levers and "brifters".
This is a guide, however with all bike parts, it is worth reading product descriptions to make sure they will suit your requirements.