The larval cases of Coleophora anatipennella, aka the Pistol Case-bearer, or at least the feeding signs it leaves behind, can be relatively easy to find at the moment, for as long as the leaves last anyway. The examples below were found on Blackthorn.
Coleophora anatipennella feeding signs on Prunus
If you do manage to find the feeding signs, and the case is not present on the leaf, it is worth a close inspection of the nearby leaves and particularly in the leaf axils, as the case can often be found here.
Coleophora anatipennella larval case on Prunus