Mundane means will entirely depend on the way the invisibility spell works, as defined by its trappings. Does it make the target transparent? Mud and blown dust to find them, and it doesn't work well in heavy rain at close range. Does it make everyone else just "not notice" them? Then some kind of hyper-awareness drug might help. Etc, etc., with the means varying by trapping and discoverable by the application of reason and common sense to the nature of the trapping.
Make sure your casters define their trappings, and the mundane countermeasures become clear.
Savage Worlds Deluxe Edition gives you a few stock spells that is designed to deal with invisible opponents, among other things:
Detect Arcana grants the ability to sense supernatural happenings, including invisible people. (I was going to say "shows their aura" or something like that, but that's just how I picture it – of course this also entirely depends on the trappings!)
Dispel cast in the right direction can force a contest to remove the invisibility. This doesn't work on natural magical effects though, so a naturally-invisible opponent is safe from Dispel.
Drain Power Points can suck out all the magical power of a caster, making them unable to maintain the invisibility. This doesn't work on natural invisibility either.
Mind Reading can force a target to "tell" you where they are.
Note that only Detect Arcana is much good for avoiding the penalties to Notice – the rest can only really be used after successfully noticing an invisible individual, and will still suffer the targeting penalty. I'd personally rule that Dispel is a bit of an exception in that it's not so much targeted at the individual as it's cast sort of vaguely toward an effect, so it doesn't need to be nearly as precise as, say, a knife strike has to be – but that's up to your particular GM, since the text doesn't say anything in particular about targeting Dispel.
And of course template area effect spells like Havoc, Pummel, and Slumber work just as well on invisible people caught in the area as visible ones, and you don't have to worry about the targeting penalty at all. You can also get creative and use something like Obscure to plunge both you and the invisible opponent into darkness, possibly negating their advantage.