Summarize this article:

Tap the Snapchat app icon, which resembles a white ghost on a yellow background. The Snapchat camera will open if you're logged in. If you aren't logged in, tap LOG IN, enter your username (or email address, or phone number) and password, and tap LOG IN. Swipe right over the Camera page to do so. You should see a list of recently snapped friends appear. Scroll down until you find the name of the person to whom you recently sent a snap. You should see an outline of a triangle and the word "Opened" below the person's name, along the time which they opened the snap listed in seconds, minutes, or hours.  If your friend opened the snap within the last minute or so, they're most likely still online. If you instead see a solid triangle with the word "Delivered" next to it, they haven't yet opened your snap.

Summary:
Open Snapchat . Scroll over to the Friends page. Find your friend's name. Look at the "Opened" timestamp for the last snap you sent to them.