forms.py

class MyForm(forms.Form):
  # ... Many fields
  
  def clean(self):
    foo = self.cleaned_data.get('foo')
    bar = self.cleaned_data.get('bar')
    if foo and bar and foo > 0 and bar < 0:
      self.add_error('bar', ValidationError('Some message', code='foo_bar_mismatch'))
    # ... Other validation rules here

tests.py

class TestMyForm(TestCase):
  def test_foo_bar_mismatch(self):
    data = {
      # .. other fields
      'foo': 1,
      'bar': -1,
    }
    form = MyForm(data=data)
    self.assertFalse(form.is_valid())
    self.assertTrue(form.has_error('bar', code='foo_bar_mismatch'))