Determine all variables needed for DMN with multiple variables in expression

How can I determine all required input variables to a DMN / DRD model?
Especially with the case that I can define input expression in the input column.

For example, if I have 2 input columns, the first column is just a variable “aaa”, and the second column is an expression “bbb + ccc > 10”. In this case, I will need to provide all “aaa”, “bbb” and “ccc” when I invoke evaluateDecision method via the Java API.