I myself am overweight, when I asked my friends they are already use to knowing that I workout so it's hard to gauge whether or not if you can really tell or not
There's not a way to tell. The only way you can tell if someone big is or has worked out is someone like The Rock or Arnold Schwarzenegger. They have so many obvious muscles that you can tell non of it would be fat for any reason.
If you're really conscientious of it though I wouldn't be walking around with food in my mouth, while holding more food, while holding a letter size mug of pop. If your doing that, I don't think anyone would have doubts. If you know what I mean. Otherwise I wouldn't worry about it.