FirstPosition

FirstPosition[expr,pattern] gives the position of the first element in expr that matches pattern, or Missing["NotFound"] if no such element is found.

FirstPosition[expr,pattern,default] gives default if no element matching pattern is found.

FirstPosition[expr,pattern,default,levelspec] finds only objects that appear on levels specified by levelspec. 

FirstPosition[pattern] represents an operator form of FirstPosition that can be applied to an expression.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.