You previous bills should be showing your internet usage, but if you are transferring to an Optus NBN plan it will be unlimited data so no need to worry.
If not Optus I would still recommend going for an unlimited data plan as the difference in cost is minimal and people are streaming video more and more so their data usage is increasing.
Unless you are an internet fiend its unlikley your internet usage on ADSL has been very much. Its just not possible to download a lot. Most NBN plans are geared to allow 200Gb or 500Gb or Unlimited. All are likely way more than you use on ADSL.