I'm going to assume you mean automated testing rather than just checking that the post request seems to work. If you do mean the latter, just check by executing the request and checking the values of the relevant Foo and Bar in a shell or in the admin.

The best way to go about sending POST requests is using a Client. Assuming the name of the view is my_view:

from django.test import Client
from django.urls import reverse

c = Client()
c.post(reverse('my_view'), data={'fid':43, 'bid':20})

But you still need some initial data in the database, and you need to check if the changes you expected to be made got made. This is where you could use a TestCase:

from django.test import TestCase, Client
from django.urls import reverse

FooBarTestCase(TestCase):

def setUp(self):
    # create some foo and bar data, using foo.objects.create etc
    # this will be run in between each test - the database is rolled back in between tests

def test_bar_not_changed(self):
    # write a post request which you expect not to change the value 
    # of a bar instance, then check that the values didn't change
    self.assertEqual(bar.value, old_bar.value)

def test_bar_changes(self):
    # write a post request which you expect to change the value of 
    # a bar instance, then assert that it changed as expected
    self.assertEqual(foo.value, bar.value)

A library which I find useful for making setting up some data to execute the tests easier is FactoryBoy. It reduces the boilerplate when it comes to creating new instances of Foo or Bar for testing purposes. Another option is to write fixtures, but I find that less flexible if your models change.

I'd also recommend this book if you want to know more about testing in python. It's django-oriented, but the principles apply to other frameworks and contexts.

edit: added advice about factoryboy and link to book

you can try putting "print" statements in between the code and see if the correct value is saved. Also for update instead of querying with "get" and then saving it (bar.save()) you can use "filter" and "update" method.

@login_required(login_url='/try_again')
def change_bar(request):
foo_id = request.POST['fid']
bar_id = request.POST['bid']
foo = models.Foo.objects.get(id=foo_id)
if foo.value > 42:
        models.Bar.objects.filter(id=bar_id).update(value=foo.value)
        #bar.value = foo.value
        #bar.save()
return other_view(request)