to check if you have aftermarket just have a look underneath the car, most of the time there is a sticker on the shock with the brand on it.
to check if your shocks are stuffed go to any pedders or traction tyres in rowville, pedders have a test it costs like $20 or something and they should be able to tell you what exactly is stuffed if anything.
traction tyres in rowville are good but they will be a bit pricey.