Caprara et al. (2005) developed a scale for measuring adults’ prosociality. It consists of 16 ordinally scaled items each with five response options. The original authors reported an internal consistency of α = 0.91. You can search the article “A New Scale for Measuring Adults’ Prosocialness” in Google Scholars and download it.

How do you conceptualize prosocial behavior, by the way?

There is social value orientation scale which is rooted in social psychology and widely used in behavioral scinces, particularly, behavioral economics. The scale measures prosocial behavior among four types of social value orientations.

Further reading may help:

Article Social Value Orientation: Theoretical and Measurement Issues...

Article Measuring Social Value Orientation

Article Eye Gaze Patterns of Decision Process in Prosocial Behavior

And papers under Sociality Project I jointly led with Mikhail Myagkov and Tatiana Sergeevna Kozitsina . Available here: https://www.researchgate.net/project/Laboratory-of-Experimental-Methods-in-Cognitive-and-Social-Sciences

Reputational and observational measures are often used to assess people's prosocial behaviors. When questionnaires are used they often raise questions about their reliability and validity.

Hello, there is also Prosocial scale in a Strengths and diffuculties questionnaire - it is often used and translated to many languages. We used it with adolescents several times (see attached for example). And the scale itself is available here: https://www.sdqinfo.org/a0.html