  We use surface brightness fluctuation (SBF) measurements to constrain the
distance to low surface brightness (LSB) dwarfs in the vicinity of M101. Recent
work has discovered many LSB candidate satellite companions of M101. However,
without accurate distances, it is problematic to identify these dwarfs as
physical satellites of M101. We use CFHT Legacy Survey (CFHTLS) data to measure
the SBF signal for 43 candidate dwarfs. The data is deep enough that we
constrain 29 of these to be unassociated background galaxies by their lack of
SBF. We measure high S/N SBF signals for two of the candidate dwarfs, which are
consistent with being at the distance of M101. The remaining candidates are too
LSB and/or small for their distances to be constrained. Still, by comparison
with Local Group dwarfs, we argue that the M101 satellite system is likely now
complete down to stellar masses of $\sim5\times10^5$ M$_\odot$. We also provide
a new SBF distance for the nearby dwarf UGC 8882, which suggests that it is
significantly outside of the virial radius of M101 and is thus not a physical
satellite. By constraining the distances to a majority of the candidates using
only archival data, our work demonstrates the usefulness of SBF for nearby LSB
galaxies and for studying the satellite systems of nearby massive galaxies.
