Fresh spackle goes on like "fresh spackle". which should be smooth... If it's a tad hard, mist it with some water and it might freshen it a little bit. If I need spackle, I just spend $4 and go buy it and use what I need. If it's questionable next time I open it, I just buy more. It's cheap Honestly, I rarely need it though.
When your in the store, pick up the tubs and feel the difference in them. The light stuff will feel empty. Always build light when you can. I stay away from putty.. it's heavy. To each his own though.