You can view your design in Sketch in two modes: vector and pixel. Which way you prefer will depend on the kind of work you do.
These modes can be toggled anytime; choose View › Canvas › Show Pixels in the menu, (or press Control-P). You can also customize your toolbar to include Show Pixels as an item.
Note: When Show Pixels is enabled, you need to zoom in past 100% in order to see the effect.
Previewing designs with pixel zoom is a great way for previewing what you may want to export. Ideal for creating icons as you can make sure shape’s bounds are sitting on full pixel values.
View Pixel Grid
The Pixel Grid allows you to differentiate individual, fuzzy pixels in low contrast that would otherwise go unseen. To enable the Pixel Grid, choose View › Canvas › Show Pixels Grid (or press Control-X). When combined with Show Pixels, any edges that do not align with the pixel grid will be visible when you zoom in past 600%.
Learn more about producing pixel perfect designs in Pixel Precision.