  We show how the pairing symmetry of superconducting states in twisted bilayer
graphene can be experimentally identified by theoretically studying effects of
externally applied in-plane magnetic field and strain. In the low field regime,
superconducting critical temperature $T_c$ is suppressed by in-plane magnetic
field $\boldsymbol{B}_{\parallel}$ in singlet channels, but is enhanced by weak
$\boldsymbol{B}_{\parallel}$ in triplet channels, providing an important
distinction. The in-plane angular dependence of the critical
$\boldsymbol{B}_{\parallel, c}$ has a six-fold rotational symmetry, which is
broken when strain is present. We show that anisotropy in
$\boldsymbol{B}_{\parallel, c}$ generated by strain can be similar for $s$- and
$d$-wave channels in moir\'e superlattices. The $d$-wave state is pinned to be
nematic by strain and consequently gapless, which is distinguishable from the
fully gapped $s$-wave state by scanning tunneling measurements.
