In order to be able to support children to acquire basic arithmetic skills in natural learning situations like play and everyday activities before they start school, it is necessary that the early childhood professionals recognize and assess these basic skills in the children’s actions and statements. This is a challenging task, because it requires applying explicit knowledge about mathematical development spontaneously in different play and everyday activities. Therefore, early childhood professionals need support in the form of formative assessment tools that facilitate precise observations of these basic skills within natural learning situations and as well a structured documentation. This paper presents two natural learning situations (a game and a picture book), which allow the assessment of early arithmetical skills, as examples of how assessment of early arithmetic skills can be implemented in daily kindergarten practice. Results of a first validation study are provided for the picture book situation.