The adult moth is hard to locate so it's best to look for the leaf mines on mistletoe, and May is apparently the ideal time. I've no personal experience of this moth; the information below is extracted from some guidance produced in England:
Mines can be found over a long period of time. At first the mine is quite inconspicuous – a small crescent marked by an entrance hole. These are almost impossible to find above eye level so a negative survey in winter should not be taken as confirmation of absence.
Depending on the season spring feeding resumes by late March. At first this takes place in a narrow gallery, which is subsequently enlarged into an inflated blister. The larva is a striking deep green colour. May is by far the best time to look as the mines are pale and conspicuous. In June the affected leaves fall off. The adult rests on the trunks of apple trees in July but this will be a less reliable way of recording the species.
Binoculars or even a telescope are useful and can reveal mines even when none have been found lower down. However, it is important to be familiar with what to look for because it can be difficult to be sure about what you’re seeing. Try looking from a number of different angles as mined leaves are easily obscured by others in the clump. It is not helpful to try and see through the leaves as they are so thick – you are looking for surface discolouration so keep the sun behind you.
Mines can occur at very low density. On a site of any great size in my experience at least half an hour is needed before you can say with any confidence that the moth isn’t present. Thicker, more succulent mistletoe is preferred to drier clumps on distressed trees.
There are potential confusions for recorders unfamiliar with leaf-mining lepidoptera. I have reared Ditula angustiorana from a spinning and suspect at least of couple of other polyphagous insects will gnaw at mistletoe leaves. Damage and wear on leaves could over-optimistically be thought to be mining activity.
The photo below shows a typical woodiana orchard. However, the moth can also survive on quite isolated clumps of mistletoe so sites are worth checking even if there is not a great abundance of the plant.
Good luck, and do let me know if you find any mines!