diff --git a/internal/util.go b/internal/util.go
new file mode 100644
index 0000000..f7e10d2
--- /dev/null
+++ b/internal/util.go
@@ -0,0 +1,26 @@
+package internal
+
+import (
+	"reflect"
+)
+
+// RemainingKeys will inspect a struct and compare it to a map. Any key that
+// is not defined in a JSON tag of the struct will be added to the extras map
+// and returned.
+//
+// This is useful for determining the extra fields returned in response bodies
+// for resources that can contain an arbitrary or dynamic number of fields.
+func RemainingKeys(s interface{}, m map[string]interface{}) (extras map[string]interface{}) {
+	extras = make(map[string]interface{})
+	valueOf := reflect.ValueOf(s)
+	typeOf := reflect.TypeOf(s)
+	for i := 0; i < valueOf.NumField(); i++ {
+		field := typeOf.Field(i)
+		tagValue := field.Tag.Get("json")
+		if _, ok := m[tagValue]; !ok {
+			extras[tagValue] = m[tagValue]
+		}
+	}
+
+	return
+}