How do I restore iOS safari history/determine when it was cleared?
There isn't a simple way to do this since the system doesn't record an audit log of changes to safari that would allow you to easily detect the when and more importantly the what changed from moment to moment.

Theoretically, the file system would have a last modified date (that can easily be changed) to indicate the last time the file was saved - but you would either need some forensics skills to build up a history and a snapshot of the filesystem going back in time - like Time Machine might provide you.

Since Time Machine doesn't exist on iOS - your best bet is to restore the entire phone from your backups and test to see if the history is correct. This is similar to building a new house because you want a new cabinet drawer in one bathroom, but there just isn't an interface (at least built by Apple) to automate what you ask.