Is this a rectangle?
Python 3, 88 \$\cdots\$ 54 43 bytes



lambda l:len({abs(sum(l)/4-z)for z in l})&lt;2


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How

Calculates the distances from the quadrilateral's centre of mass to all of its vertices and tests if they're equal.  

Input The four vertices as a sequence of complex numbers in any order.
Output True/False