Yes, you can see if someone downvotes a post. But you can't see who downvoted the post.
Other than the negative number seen, you can tap the number that represents the number of votes received you'll see a list of just the people who gave an upvote. Make a count of the people on the list and compare the count with the actual number you see. Of course, a number smaller than the count of people on the list means there we some downvoted
btw done with App only