Detect when a SwiftUI sheet is swiped away

onDismiss is enough, you need just to add another variable and check on it.

For example: let's declare an enum

enum SheetAction {
    case cancel
    case nothing
}

Then in your Main View:

@State var sheetAction: SheetAction = SheetAction.nothing

Like that the default action which cause the dismiss will be nothing which means a swipe down, not a button.

And pass it to your SheetView

.sheet(isPresented: self.$sheetActive, onDismiss: {
    if sheetAction == .cancel {
        // dismissed using cancel button
        dismissTxt = "Cancel"
    }else if sheetAction == .nothing {
        // dismissed by swipe down
        dismissTxt = "Swipe down"
    }
}, content: {
    SheetView(isActive: self.$sheetActive, action: self.$sheetAction)
})

When the user click on Cancel button, change its value. And don't miss to change the value to nothing each time the SheetView appears.

The final code:

enum SheetAction {
    case cancel
    case nothing
}

struct CustomView: View {
    
    @State var sheetActive = false
    @State var sheetAction: SheetAction = SheetAction.nothing
    
    @State var dismissTxt = ""
    var body: some View {
        VStack {
            Text(dismissTxt).foregroundColor(.gray)
            Button(action: {
                self.sheetActive = true
            }, label: {
                Text("Button")
            })
        }.sheet(isPresented: self.$sheetActive, onDismiss: {
            if sheetAction == .cancel {
                // dismissed using cancel button
                dismissTxt = "Cancel"
            }else if sheetAction == .nothing {
                // dismissed by swipe down
                dismissTxt = "Swipe down"
            }
        }, content: {
            SheetView(isActive: self.$sheetActive, action: self.$sheetAction)
        })
    }
}

struct SheetView: View {
    @Binding var isActive: Bool
    @Binding var action: SheetAction
    var body: some View {
        NavigationView {
            VStack {
                Text("Sheet content")
            }
            .navigationBarItems(leading: Button(action: {
                // Code that dismisses sheet
                self.action = .cancel
                self.isActive = false
            }) {
                Text("Cancel")
            }, trailing: Button(action: {
                // Code that adds something to something
            }) {
                Text("Add").bold()
            })
        }.onAppear {
            action = .nothing
        }
    }
}