OnlyTraced answers the question you won't ask them

Paste one Snapchat username and their Best Friends and snap score show up on screen, checked privately.

Nothing to download. It opens right in your browser.

What would I actually see?

OnlyTraced

@their.username

Friendship profile

Snap score

Best Friends

And after I tap the button?

OnlyTraced

Three screens after you tap

A box for one usernameYou will see one field asking for their Snapchat username, nothing else.
The check on that nameYou will see the check run on that name, on the same screen.
Their list and their scoreYou will see their Best Friends and snap score laid out in front of you.

Wait, what is the Best Friends list actually?

OnlyTraced

Snapchat keeps a Best Friends list for every account: the people it snaps and chats with most, up to eight of them.

Friends can see whether they are on it, but nobody else sees the whole list.

A yellow heart marks two people who are each other's #1 Best Friend, a smiley marks someone else on the list.

Snapchat Support, "How do Best Friends work on Snapchat?": up to eight Best Friends, updated often.
Snapchat Support, same page: people may see if they're among your Best Friends, no one else can view the list.
Snapchat Support, "What do my Friend Emojis mean on Snapchat?": the yellow heart and smiley face.

OnlyTraced

One username is all it takes to see who sits at the top of their list.