Let’s start by detecting the item through its tag, but first, let’s get an iron sword to use as an example.
give @a iron_sword[custom_data={Teste:1}]
Choose a tag using custom_data, then place a Repeat command block set to Always Active with the following command:
execute if items entity @p weapon.mainhand minecraft:iron_sword[custom_data={Teste:1}]
Basically, this will detect whether the nearest player to the command block is holding an iron sword with the tag Teste:1b in their main hand.
The idea is the same for detecting by name. Let’s once again give the player a sword, but this time with a custom name:
give @a iron_sword[custom_name='[{"text":"Teste"}]']
Then place this command in a Repeat command block set to Always Active:
execute if items entity @p weapon.mainhand minecraft:iron_sword[custom_name='[{"text":"Teste"}]']
If you’re working in multiplayer, use execute as @a run at the beginning of the command to apply it to all players,
and replace @p with @s to target the player executing the command.