For starters, it is very uncommon to come across cable that has no marking at all, and I would be very careful when using such cable for any electrical installation, since you can not be certain of what temperature range and voltage level it can be good for, in addition to the question of cross-section area. It could even be that the manufacturer has detected some manufacturing flaws, and therefore decided not to mark it, and simply sell it to a scrap metal dealer instead of bringing it to the market.
Secondly, you have at least three fairly accurate methods to determine thee cross section area (CSA), but none can be used if there are any variations in the cross-section area (which could be one reason why it is not marked).
Method 1): Measure the core diameter, and calculate the CSA from that, assuming the cross-section is circular. This method is not accurate for sector-shaped cable cores.
Method 2): Cut off a length of the cable, and strip it of all insulation, then you can weigh the cut-off length, and calculate the CSA from the specific density of the conductor material which should be either Cu or Al. Any alloys would have densities vary close to the base material.
Method 3): Measure off a reasonably long length of the cable, and measure the resistance of the length. Then the CSA can be determined from the specific conductivity of the material. Only a DC measurement will be reliable.
If none of these methods give a result close to one CSA referred to in AS 3008, you may have come across a cable made to an American standard, so you will have to look up which dimension comes closes to your measurement.