if you want to find particular form is already opened or active. i thing the following may help you.
List<MyForm> CurrentActiveForms = System.Windows.Forms.Application.OpenForms.OfType<MyForm>().ToList();
if(CurrentActiveForms.Count == 0)
MyForm DesireForm = CurrentActiveForms.FirstOrDefault(I => I.MyCondition == myCondition);
MyForm DesireForm = CurrentActiveForms;
if (DesireForm != null || DesireForm.IsDisposed == false)