  We describe an efficient algorithm which, given a principally polarized
(p.p.) abelian surface $A$ over $\mathbb{Q}$ with geometric endomorphism ring
equal to $\mathbb{Z}$, computes all the other p.p. abelian surfaces over
$\mathbb{Q}$ that are isogenous to $A$. This algorithm relies on explicit open
image techniques for Galois representations, and we employ a combination of
analytic and algebraic methods to efficiently prove or disprove the existence
of isogenies. We illustrate the practicality of our algorithm by applying it to
1 440 894 isogeny classes of Jacobians of genus 2 curves.
