For a genus 2 curve over the rational field whose Jacobian A admits only trivial geometric endomorphisms, Serre's open image theorem for abelian surfaces asserts that there are only finitely many primes ell for which the Galois action on ell-torsion points of A is not maximal. Building on work of Dieulefait, we give a practical algorithm to compute this finite set. The key inputs are Mitchell's classification of maximal subgroups of the projective symplectic group of 4 x 4 matrices over the finite field of ell elements, sampling of the characteristic polynomials of Frobenius, and the Khare--Wintenberger modularity theorem. The algorithm has been submitted for integration into Sage, executed on all of the genus 2 curves with trivial endomorphism ring in the LMFDB, and the results incorporated into the homepage of each such curve. This talk is based on joint work with Barinder S. Banwait, Armand Brumer, Zev Klagsbrun, Jacob Mayle, Padmavathi Srinivasan, and Isabel Vogt.