I'm pretty particular when buying tv shows or anime so I was just wondering if there's any way of knowing whether or know an anime set comes with a slip cover over the case or not?
No real way of knowing for sure, but from personal experience the vast majority of things that I've bought from FUNimation (outside of S.A.V.E. releases) have come with slipcovers.
So unless it's a S.A.V.E. or Viridian edition, you should be safe.
It should be noted some SAVE editions do have slip covers. But yeah all of my non-save editions have slip covers.
Though I wish more of the slip covers had different art than the actual covers. Only my copies of Trigun and Summer Wars do. Not sure if there are anymore that have different art.
Are slips more desirable? I find they wear pretty fast and look like crap on my shelf. Most funi titles I have all come with slips except for save.