  At z>1, the distinction between merging and 'normal' star-forming galaxies
based on single band morphology is often hampered by the presence of large
clumps which result in a disturbed, merger-like appearance even in rotationally
supported disks. In this paper we discuss how a classification based on
canonical, non-parametric structural indices measured on resolved stellar mass
maps, rather than on single-band images, reduces the misclassification of
clumpy but not merging galaxies. We calibrate the mass-based selection of
mergers using the MIRAGE hydrodynamical numerical simulations of isolated and
merging galaxies which span a stellar mass range of
$10^{9.8}$-$10^{10.6}M_{sun}$ and merger ratios between 1:1-1:6.3. These
simulations are processed to reproduce the typical depth and spatial resolution
of observed HUDF data. We test our approach on a sample of real z~2 galaxies
with kinematic classification into disks or mergers and on ~100 galaxies in the
HUDF field with photometric/spectroscopic redshift between 1.5$\leqslant z
\leqslant$3 and $M>10^{9.4}M_{sun}$. We find that a combination of the
asymmetry $A_{MASS}$ and $M_{20, MASS}$ indices measured on the stellar mass
maps can efficiently identify real (major) mergers with $\lesssim$20%
contamination from clumpy disks in the merger sample. This mass-based
classification cannot be reproduced in star-forming galaxies by $H-$band
measurements alone, which instead result in a contamination from clumpy
galaxies that can be as high as 50%. Moreover, we find that the mass-based
classification always results in a lower contamination from clumpy galaxies
than an $H-$band classification, regardless of the depth of the imaging used
(e.g., CANDELS vs. HUDF).
