You got the basic idea that trough is an area of disturbed weather but for you can connect the isobars (area's of equal pressure) you need buoy readings. Most of the upper level air features are found at the 500 mb level because pretty much that's were they reside. So you can see the trough at a 500 mb level.
Here's an excellent website in teaching some basic things from geopontential heights to hurricanes ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/geos.rxml
Just click online guides and choose the topic of your choice.

Edit: Just copy and paste don't add "www" or "com"