Minecraft – ny way to test if a Players inventory is not full in Minecraft

• In the top left command block: fill ~ ~-1 ~ ~ ~-1 ~ wool 14
• In the bottom left command block: fill ~ ~1 ~ ~ ~1 ~ redstone_block
• In the center-left command block: testforblocks ~ ~-1 ~ ~ ~-1 ~ [x2] [y2] [z2] where [x2] [y2] and [z2] are the coordinates of the target dropper.

The dropper on the top is completely unnecessary and is only there because I used this in a tutorial. In the far right command block would be the command you want executed when the two droppers match.

We're going to use the fact that you are checking for boots to our advantage, since boots are not stackable and there can normally only be 0 or 1 item in that slot.

Set up a dummy scoreboard objective called boots and run the following two Commands on a setblock/fill clock.

scoreboard players set @a boots 0
scoreboard players set @a boots 1 {Inventory:[{Slot:100b,Count:1b}]}

For example, you can use a setup like this one.

This will set the boots score to 1 for everyone having exactly one item in their foot inventory slots. From there, you can use @a[score_boots=0] to detect barefoot players.

A general answer for every inventory slot is more difficult, because checking for Count:0b does not work, since for the game, "no item" is different from "0 items of any kind".