The device datasheet specifications provide voltage switching levels based on JEDEC specifications, and the minimum current the output buffer will drive at those voltage levels. Altera® devices will drive at least the minimum amount of current across all available programmable drive strength and series on chip termination (OCT) settings.
You can determine the actual current source and sink capabilities for each programmable drive strength and series OCT setting by viewing the output buffer IV curve using IBIS models. There are free IBIS viewers available from 3rd party vendors.
IV curves are generally based on a no load condition unless otherwise specified. For example, the PCI specification requires IV curves to be generated with a 10pF load. To view the output buffer response under specific loading conditions, a full IBIS simulation would be required.
Download Altera IBIS models for your device family, or you can generate design specific IBIS models using the Quartus® II software if supported for your device family.