Rather than asking "will it scale?", I suggest asking "could it scale?". There's no point in doing up-front work to make something scalable if you don't know if it will be needed, but it's also important not to design features that absolutely could not scale under any circumstances. If you can think to yourself "if I need to scale this, I've got a rough idea of how I would do it" you'll probably be fine. And if not... once again, not being able to scale fast enough is a great problem to have (telling an investor "I don't have time to talk to you, our server's can't keep up with all of our new signups" is a great pitch).