First select your player, (from squad screen or whatever), then select their training overview. You should see a blank graph with no lines on and a button marked "None." If you press on "none" you will get a drop down menu of all of the relevant training categories.
Select the one you're interested in and the appropriate individual player stats that are affected by that training category will all be highlighted in different colours, each colour representing a line on the graph.
Look at these lines and hope that they are all heading upwards!