Showing which way line features are oriented in QGIS?

Simplest way is to style the line. Add a new symbol layer to your line and change the 'Symbol Layer Type' to marker line. Select 'Marker' in the style tree and select a triangle as your default marker symbol. I believe 'triangle 2' is your best bet. Accept these changes and all your lines will now have arrow showing the direction of the line. You can mess around with distances and marker placement to make it look better but this essentially show you where your lines are going.

Following up on Knightshound's answer, it looks like there were some slight changes to the marker symbols in QGIS 3. After selecting 'Marker' in the style tree, you may not see 'triangle 2' as an option in the list of symbols. If so, look at the drop down list labelled 'Symbols in'. In my version, 3.0.3, this defaulted to 'Favorites'. If you change it to 'All Symbols', you should see 'triangle 2'.