Depends on how you have your iPhoto setting set up. If under Preferences in iPhoto you have the options set up to automatically import your Photo Stream photos, then they should already be in monthly Events in iPhoto:
If you do not have these options set up, but if you have been regularly importing from your Camera Roll, then those also should be showing in Events in iPhoto. If you see your photos in either Events or Photos (selected from the top of the iPhoto sidebar), then they are safely stored in iPhoto, and you can delete them from your device.
If you do not see them in the Events or Photos views, then you would need to turn on the Photo Stream options above, and see what gets imported. Depending on the dates of the photos in your Photo Stream on your device, those photos may or may not still be in Photo Stream in iCloud. iCloud will keep the last 30 days of photos up to 1000 photos. But your device will keep Photo Stream photos on the device until you reset Photo Stream on the device.
I have photos in my iPhone Photo Stream that go back to last April....but, if I turned off Photo Stream in iCloud, most of them would be deleted because iCloud itself only has the last 30 days worth.
If all of the photos in your Photo Stream are not imported to your computer, you can put them in a Shared Photo Stream and send it to yourself, then import them to your Library right on the Mac.
Cheers,
GB