Is this string a square?

Python 2, 52 bytes

x=input().split('\n')
print{len(x)}==set(map(len,x))

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Brachylog (2), 3 bytes

ṇẹṁ

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Full program. Outputs true. for truthy, false. for falsey.

Explanation

ṇẹṁ
ṇ     Split {standard input} into lines
 ẹ    Split {each line} into basic elements {in this case, characters}
  ṁ   Assert that the result is square

I was a bit sceptical about the usefulness of the ṁ builtin when it was added, but I can't really deny that it's helpful here…

Brachylog (2), 7 bytes

ṇẹ.\l~l

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Non-builtin solution. Still beats all the other entries, as of the time of writing. EDIT: Not quite, the equal-length Jelly entry got in while I was writing this, and beats it via the timestamp tiebreak.

Explanation

ṇẹ.\l~l
ṇ         Split {standard input} into lines
 ẹ        Split {each line} into basic elements {in this case, characters}
   \l     Assert that the result is rectangular, and the number of columns
  .  ~l     is equal to the number of rows

JavaScript (ES6), 46 45 bytes

s=>!(s=s.split`
`).some(x=>x.length-s.length)

• 1 byte saved thank to ETHproductions

Explanation

1. Split the string to an array on newlines.
2. Loop over the array.
3. Subtract the length of the array from the length of each line.
4. If a non-zero (i.e., truthy) value is returned for any line, the string is not square.
5. Negate the result of the loop to get true for square and false for not.

Try it

f=
s=>!(s=s.split`
`).some(x=>x.length-s.length)
oninput=_=>o.innerText=f(i.value)
o.innerText=f(i.value=`foo
bar
baz`)

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