Check out https://github.com/odoo/odoo/blob/302037f48abbdfb146a41a128a039174d8844ab1/odoo/models.py#L665

You can leverage this method to get the information you need. 

Sample code would look something like:

fields_with_onchange = ""
for field in model._onchange_methods:
  fields_with_onchange += field + ", "

If you run this for the sale.order model, you get:

fiscal_position_id, commitment_date, company_id, partner_shipping_id, partner_id, sale_order_template_id, user_id, team_id,