Richard,
The first thing you should notice is the cheese getting soft. Press in the middle of the cheese regularly and get used to the feel/resistance. As it ages it will soften considerably.
Secondly, you will start to get a slight darkening on the rind, often on the corners. This can be accompanied by a slight smell of ammonia, noticable when you open the container/wrapping. The ammonia smell is caused by tghe breakdown of the proteins, and gets stronger as the cheese gets over-ripe.
You might also notice the cheese "shrinking' away from the rind a little. This occurs as the paste gets softer. You can usually see this on the corners and across the top, as the cheese starts to sink inwards.
Just keep looking at it regularly. If you start to smell ammonia, it is pretty well ripe. Good luck, keep us posted with photos!