You may want to assure the validity of tool content by an expert, perform exploratory and confirmatory factor analysis, and correlate its scores with another valid tool. This is not a simple process.
There have been a number of discussions here on this topic. Use the Search function for Questions to look up "validity scales" and "validity questionnaire."
However, it depends what you are trying to do. For example, sometimes construct validity if most important (if you are measuring a concept); sometimes predictive or concurrent validity will suffice (if there is a criterion you have to predict).