How to tell if a certain item is in a player inventory?

You can use /clear<player> netherite_sword -1 0. The -1 matches all data values* and the 0 specifies the maximum number of items to clear (so none will be cleared). If the player has at least 1 netherite sword in their inventory, the command will output a success.

The success can be detected using a chain command block. Put the /clear any command block facing a conditional, always active, chain command block with the command that you want to execute if the player has a netherite sword.

^{* I'm not sure if a sword has any data values (e.g. for durability) so just match all data values instead of 0 just in case.}