Finding the cone from something or someone's point of view?

[acoustic anchor]

There is a method in the camera class that will return a Boolean representing whether an object is inside the view frustum or not

A naive implementation ( or a suitable one depending on the context ) may be just to loop through every instance in the map / workspace and use this method to test wherever it is in the view frustum or not.

Well the naive approach works, looping through every possible instance in workspace is a lot of work, especially as your game scales

so it might be best to invest in some sort of spatial partitioning algorithm to cut the amount of instances you need to check

You could also look into things like using a region3 which is fitted to the view frustum and grabbing all the parts in it as well