FCLoopSamePropagatorHeadsQ[exp] returns True if the FeynAmpDenominator of exp contains only propagator denominators of the same type (e.g. only StandardPropagatorDenominator or only CartesianPropagatorDenominator).
FCI@SFAD[q, q - p]
FCLoopSamePropagatorHeadsQ[%]\frac{1}{(q^2+i \eta ).((q-p)^2+i \eta )}
\text{True}
FeynAmpDenominatorCombine[CFAD[q, q - p] SFAD[l, l + k]]
FCLoopSamePropagatorHeadsQ[%]\frac{1}{(q^2-i \eta ).((q-p)^2-i \eta ).(l^2+i \eta ).((k+l)^2+i \eta )}
\text{False}