Recognize Fillet

First of all there is no trivial solution for this unless you write your program to be feature based.(i.e. tag a fillet as a "fillet" while it is created).

For simple fillets you can try this.

First check if the assumed fillet surface is cylindrical or spherical (Face to face fillets are cylindrical and corner fillets are spherical). Now to check if the cylindrical surface is really a fillet check for two faces linked to it to be planar. This can help you only to check if the features are not very complicated like in female radius connected to male radius etc. where you will find cylindrical surface attached with cylindrical surface.

There are many possibilities that a fillet surface may have different kind of surfaces attached to its boundaries. Finally a fillet results in a surface which has radius of curvature which can be evaluated on its underlying geometry.

However, assuming that a simple box is filletted and you want to detect the fillets, the above method will be helpful.

But anyway you can always check for cylindrical or spherical surfaces.

Best Luck!