Minecraft – How to test for another player in a radius

What you could do to make your testing as real as possible is target entities with the @e selector in replacement of real players, and try testing your command blocks with entities like villagers.

For an example if you were trying to tp a player with /tp @p (Coordinates) You could replace @p or @a with @e to teleport entities.

@e should be effective for most contraptions involving players.

Note: This is how I test my contraptions unless I'm with a friend.

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.