FCLoopPropagatorsToLineMomenta[{prop1, prop2, ...}] is an auxiliary function that extracts line momenta flowing through the given list of propagators.
Overview, FCLoopIntegralToGraph, AuxiliaryMomenta.
FCLoopPropagatorsToLineMomenta[{SFAD[{q + l, m^2}], SFAD[{p, -m^2}]}, FCE -> True]\left( \begin{array}{cc} l+q & p \\ -m^2 & m^2 \\ \frac{1}{((l+q)^2-m^2+i \eta )} & \frac{1}{(p^2+m^2+i \eta )} \\ \end{array} \right)
FCLoopPropagatorsToLineMomenta[{CFAD[{{0, 2 v . (q + r)}, m^2}]}, FCE -> True,
AuxiliaryMomenta -> {v}]\left( \begin{array}{c} q+r \\ m^2 \\ \frac{1}{(2 ((q+r)\cdot v)+m^2-i \eta )} \\ \end{array} \right)
Reversed signs are also supported
{SFAD[{I (q + l), -m^2}], SFAD[{I p, -m^2}]}
FCLoopPropagatorsToLineMomenta[%, FCE -> True]\left\{\frac{1}{(-(l+q)^2+m^2+i \eta )},\frac{1}{(-p^2+m^2+i \eta )}\right\}
\left( \begin{array}{cc} l+q & p \\ m^2 & m^2 \\ \frac{1}{(-(l+q)^2+m^2+i \eta )} & \frac{1}{(-p^2+m^2+i \eta )} \\ \end{array} \right)
FCLoopPropagatorsToLineMomenta[{SFAD[{I (q + l), -m^2}], SFAD[{I p, -m^2}]},
FCE -> True] // InputForm{{l + q, p}, {m^2, m^2}, {SFAD[{{I*(l + q), 0}, {-m^2, 1}, 1}],
SFAD[{{I*p, 0}, {-m^2, 1}, 1}]}}FCLoopPropagatorsToLineMomenta[{SFAD[{{I p1, -2 p1 . q}, {0, 1}, 1}]},FCE -> True]\left( \begin{array}{c} \;\text{p1}+q \\ 0 \\ \frac{1}{(-\text{p1}^2-2 (\text{p1}\cdot q)+i \eta )} \\ \end{array} \right)