The most reliable difference between the Willow Tit (Poecile montana) and the Marsh Tit (Poecile palustris) is the white patch at the base of the upper mandible of the Marsh Tit.
Willow Tits have an all black bill with no pale bill spot.
However, the March Tit's white patch is not always clearly visible.
Another reasonably reliable difference is the cheek and face pattern of these two birds.
Whitish ear-coverts contrasting with a cold grey-brown wash on the neck sides and rear of the cheek patch is strongly indicative of a Marsh Tit.
In other words: many Marsh Tits appear to have a much smaller white ‘face’ than Willow Tits.

Grlja Waterfall area, Prokletije National Park, Montenegro, 27 April 2023