What is the right way to validate if an object exists in a django view without returning 404?

You can also do:

if not RealEstateListing.objects.filter(slug_url=slug).exists():
    # do stuff...

Sometimes it's more clear to use try: except: block and other times one-liner exists() makes the code looking clearer... all depends on your application logic.

I would not use the 404 wrapper if you aren't given a 404. That is misuse of intent. Just catch the DoesNotExist, instead.

try:
    listing = RealEstateListing.objects.get(slug_url=slug)
except RealEstateListing.DoesNotExist:
    listing = None

listing = RealEstateListing.objects.filter(slug_url=slug).first()