Without reading the datasheet, you can't know for sure, but if this is a device designed to accept a heatsink, it's safest to assume that any maximum power or current ratings assume a huge heatsink.
If you need to calculate the maximum current with a smaller heatsink, work backwards. First calculate the power dissipated at a given current. This will vary from device to device based on how it was manufactured. What semiconductor was used? What's it's geometry? How was it doped? The datasheet won't answer these questions, but it will tell you the practical consequences that result.
Once you know the power you must handle as heat, find all the thermal resistances from the junction inside the device to the ambient air. The datasheet should give a junction-to-case thermal resistance, and your heatsink will specify its own resistance, given natural convection or forced air. There is some additional resistance from the case to the heatsink; if the datasheet doesn't give a value, then \$ 0.5 ^\circ C / W \$ is a good estimate for a TO-220 with thermal grease.
Add all the thermal resistances together, and calculate the temperature rise above ambient by multiplying that total thermal resistance by the previously calculated power. Add this temperature rise to your maximum ambient temperature and compare this against the maximum junction temperature, also specified in the datasheet.