Basically, you can simply insert the SIM card into any mobile phone and call yourself. This way, you immediately see which number is being displayed. It is important that the phone into which the SIM card is inserted has the setting for caller ID suppression disabled.
Depending on which SIM card you are using, you can usually find the phone number here:
on the packaging of the SIM card
on the plastic card from which you broke out the SIM
in the order or activation email from your provider
in your contract documents