If this is a metal frame,there might be a weep hole on the back side of the chainstay....Use compressed air in the weep hole and soapy water on the crack.....if you see bubbles......not good.

You may have to pull the bottom bracket out to put the air in the tube from there or plug it if it has a weep hole.

Die check works but it has to be clean (as in no paint) for it to work well.You can get false readings from penetrent getting under paint edges.