For variable-state slots, you have to know what the positive point is for that model of machine. For example, on the Piggy Bankin' slot machine, I think it becomes positive when there are about 40 credits in the bank. At that point the player is supposed to play one coin at a time until the bank is hit. The book Robbing the One-Armed Bandits by Charles Lund (1999) covers specific positive points for various machines, however many of the machines covered in that book are now hard to find.
As for how to determine when a progressive jackpot is unusually high, you'll either have to observe it over a long period of time or find someone who has done the same. For example, SlotCharts.com keeps data on progressive slots at online casinos. But even when a progressive slot is unusually high, it's impossible to know at what point it becomes high enough to be a positive-expectation game without knowing how the probabilities on the machine are programmed. In my section Deconstructing Megabucks I attempt to figure out when the jackpot is large enough to have a player advantage.
Update: Since this question was published, SlotCharts.com is blocked to U.S. traffic.