import scala.reflect.runtime.universe._
import scala.reflect.runtime.{currentMirror => cm}
	
class A {
  var x: Int = 42
}

val a = new A

val im: InstanceMirror = cm.reflect(a)
val cs = im.reflectClass.symbol
val f = cs.typeSignature.declaration(newTermName("x")).asTermSymbol
val fm: FieldMirror = im.reflectField(f) 
fm.field.isVariable

scala> fm.field.isVariable
res0: Boolean = false

/** Does this symbol represent a mutable value?
  * If yes, `isTerm` and `isValue` are also guaranteed to be true.
  */
def isVariable: Boolean

scala> fm.field.flags.containsAll(Flag.MUTABLE)
res174: Boolean = false