  The population of massive black holes (MBHs) in dwarf galaxies is elusive,
but fundamentally important to understand the coevolution of black holes with
their hosts and the formation of the first collapsed objects in the Universe.
While some progress was made in determining the X-ray detected fraction of MBHs
in dwarfs, with typical values ranging from $0\%$ to $6\%$, their overall
active fraction, ${\cal A}$, is still largely unconstrained. Here, we develop a
theoretical model to predict the multiwavelength active fraction of MBHs in
dwarf galaxies starting from first principles and based on the physical
properties of the host, namely, its stellar mass and angular momentum content.
We find multiwavelength active fractions for MBHs, accreting at typically low
rates, ranging from $5\%$ to $22\%$, and increasing with the stellar mass of
the host as ${\cal A} \sim(\log_{10}M_{\star})^{4.5}$. If dwarfs are
characterized by low-metallicity environments, the active fraction may reach
$\sim 30\%$ for the most massive hosts. For galaxies with stellar mass in the
range $10^7<M_{\star} [M_{\odot}]<10^{10}$, our predictions are in agreement
with occupation fractions derived from simulations and semi-analytical models.
Additionally, we provide a fitting formula to predict the probability of
finding an active MBH in a dwarf galaxy from observationally derived data. This
model will be instrumental to guide future observational efforts to find MBHs
in dwarfs. The James Webb Space Telescope, in particular, will play a crucial
role in detecting MBHs in dwarfs, possibly uncovering active fractions $\sim 3$
times larger than current X-ray surveys.
