Deciding if a variety forms an algebraic group

Let $n$ be a positive integer and let $f_1, ldots, f_r$ be polynomials in $n^2$ indeterminates over an algebraically closed field $K$. We describe an algorithm to decide if the invertible matrices contained in the variety of $f_1, ldots, f_r$ form a subgroup of $GL(n,K)$; that is, we show how to decide if the polynomials $f_1, ldots, f_r$ define a linear algebraic group.