This is a difficult task if you've never used labview before. You should probably start learning some labview basics, e.g. by completing the tutorial. You need to be familiar with the concept of dataflow, controls, indicators, loops, etc.
To see what a complicated VI actually does can be difficult, even to the expert. It also depends on the programmings style of the original author.
(0) Is the diagram well structured or just a pile of wires and nodes?
(1) What is the file name? It often hints at the functionality.
(2) Look at the front panel. What are the names of the main controls and indicators. Does it look like a polished user interface, or more like a subVI.
(3) Look at the diagram. Are there comments (blocks of text explaining sections). How abou
t "file..VI properties...documentation"?
(4) Look at the connector? What are the inputs and outputs?
(5) Maybe run it after manually entering values into the controls.
(6) Enable execution highlighting and run it while watching the diagram. Place some probes.
(7) Try to contact the original author.
(8) Give up and post it here.
If the code is not well documented and structured, it's probably not worth the effort. It is unlikely that it has functionality useful for you.
Programming styles vary dramatically. Look for example at some of the entries of the
first coding challenge. All VIs do exactly the same, but their diagrams are very different.