how do I know if photos from mac have been exported to iCloud successfully?

You can go to iCloud.com and look at the pictures there.

You can also look at at the very bottom of the Library view where

it gives the progress of the upload.

I am not quite sure what you mean by "exported to iCloud"? Have you been exporting the photos from Photos to folders on iCloud Drive, or are you syncing with the Photos Library of your Mac with iCloud Photos and want to know, if the upload to iCloud Photos is complete?

Photos does not really have a status indication showing that the upload has finished. "Synced with iCloud just now" usually means that photos is no longer uploading, if there are no items shown as "Unable to upload". I would compare the number of photos and videos at www.icloud.com to the number of items on your Mac to be sure.