The most reliable difference between de Marsh Tit (Poecile palustris) and the Willow Tit (Poecile montana) 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, this white spot is not always clearly visible, like on the above photo.
Note the cheek pattern on the bird: 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.

Eifel National Park, Dreiborn Plateau, North-Rhine Westphalia, Germany, 9 August 2022