Minecraft – Check if player is not in the air or on a specific block

It's very difficult to see whether someone has an object anywhere in their inventory. That would take 36 command blocks, one for each slot! You can, however, take the object, then see if taking the object worked.

The Commands

• Set up a fast clock (but not as fast as a fill clock)
• One command block does /clear @p iron_ingot 0 1 (clears 1 iron ingot from the nearest player. You can choose which player, but it must be exactly one. Also, you must use the same selector throughout all these commands.)
• Run a comparator from that command block to another command block running this command: /scoreboard players add @p ironIngots 1 (Increases the player's ironIngots score by 1. Make sure you create ironIngots first!)
• Create another command block that does whatever you want. In the case of giving them a diamond, you would do this: /give @p[score_ironIngots_min=10] diamond. Note the target selector; it gives the player the diamond only if their ironIngots score is 10 or greater.
• The last thing you want to do is take the ironIngots score away so they don't keep getting items. Just run /scoreboard players remove @p[score_ironIngots_min=10] ironIngots 10. This removes 10 iron ingots from the player if they have an ironIngots score of 10 or greater. Any player who matches this will have received a diamond in the previous step.

How to Set Them Up

The commands listed above must run in order. It is actually very simple to put them in order. Just open the debug menu and look at the stylish new crosshair. Align the command blocks along either the green or red line so that one of those lines points down the first to the last command block.

Notes

• You don't have to press a button to use this, you just have to be the nearest player. You can change the command to the nearest player within 10 blocks, nearest player on a team, etc.
• The commands will take a short amount of time to finish, but for 10 iron ingots, it will only take half a second. It's one tick (1/20 of a second) per item taken away.
• It will take away iron ingots even if you don't have ten. However, you can submit one, then later submit five, then submit four more later, and it will still give you the diamond. You can also submit 14 at a time and the extra 4 will be saved for later.

In the future, I recommend checking the wiki before posting a question here.

This is entirely possible using the execute command and target selector variables and arguments.

Executing commands at the player

The execute command executes commands on an entity. The @p selector selects a single player.
The name= argument only selects entities with a specific name.
So execute @p[name=<your username here>] ~ ~ ~ <command> will run <command> at your location.

testfor the nearest entity.

The @e selector finds entities of any type.
The c= argument selects a certain count. If this number is positive, it will select these in order from closest to furthest. c=1 will select the nearest entity.
The rm= argument selects only entities outside of a radius. If this is set to 0, it will select any entity that is not the executing entity.
So testfor @e[c=1,rm=0] would find the nearest entity to the executor of the command.

Putting it together

So, the first command will execute the second command at the player, meaning your finished command is

/execute @p[name=] ~ ~ ~ testfor @e[c=1,rm=0]

Closing comments/notes

As far as using testfor goes, I'd recommend against it. The scoreboard is almost always more useful, since you can use things like tag to mark the entity you found.