We evaluate the contribution to the X(3872) width from a triangle mechanism in which the X decays into DD¯ - cc, then the D(D¯ ) decays into Dπ (D¯ π) and the DD¯ fuse to produce ππ. This mechanism produces an asymmetric peak from a triangle singularity in the ππ invariant mass with a shape very sensitive to the X mass. We evaluate the branching ratios for a reaction where this effect can be seen in the B→ Kπππ reaction and show that the determination of the peak in the invariant mass distribution of ππ is all that is needed to determine the X mass. Given the present uncertainties in the X mass, which do not allow to know whether the DD¯ state is bound or not, measurements like the one suggested here should be most welcome to clarify this issue.