Step 1: Ask for their number
If you're too shy to suggest taking the conversation with your potential date to text, this may be a non-starter. But you're going to need their digits to move forward, so be bold, friends!
Step 2: Find them on Snapchat
Once you have their contact info in your phone, open Snapchat and go to the Add Friends menu. "Quick Add" lists your newly added contacts that are using Snapchat, and you'll be able to see their username without actually adding them.
Step 3: Get their username and start digging
This is where luck comes in. Now that you know their Snapchat username, try searching for it on Instagram or Twitter. Unless they're totally oblivious to the whole "personal brand" thing, odds are they use the same username across all platforms.