I didn't come up with the quoted material below, it's courtesy of Clint Black the Savage Worlds Brand Manager, I'm just posting it here because it's buried in the Pinnacle forums archives and should get out in the light of day from time to time.
Encounter design in say DnD old was very much a question of experience and judgement. The Challenge Ratings were pretty awful at determining what would be a interesting but doable encounter for players. Over time you could come up with reasonable guesses which got better the longer you DM'd the game and of course you learned quickly how to 'fudge' an encounter by adjusting hit points on the fly on your creatures, hiding your die rolls etc. With DnD 4E it's significantly better because the game has a much stronger basis in math rather than "gee that would be neat" as a design philosophy. 4E was the first game I ever felt I had no reason to fudge a die roll or adjust numbers of any kind during an encounter.
With Savage Worlds which I'm currently using, encounter design is back to educated guesswork and general rules of thumb. Toss in the fact that Savage Worlds is very much 'swingy', the damage output can have serious spikes to it thanks to exploding dice and it gets a little more tricky. With other systems with huge-mongus hit point pools like a DnD system fights are guarenteed to last several rounds and the pace is slow, its like the players are sculptors with small rasps gannging up on a big block of marble. Eventually they're going to turn it into something more to their liking but it's going to take time. With savage worlds its as if they're sculptors with jack hammers only the jackhammers randomly get turned on. They might be chipping away at something for awhile or the jackhammer might suddenly get flipped on and bam! the marble block is laying broken and bleeding on the floor.
My personal rule of thumb so far is for an 'average' encounter150% of the group size in Extras and 1 Wild Card per 2 players seems to be a reasonable encounter, tough but doable. This assumes the Extras and WC's are within a die size of the players average attack die and their toughness is roughly equal to the players average damage output per attack. I also tend to wave the opponents so that they show up over the course of a round or two, this allows for easy fudging if things are going too easily or too hard for the players. Since I roll dice in the open I can't fudge that way. Another way to fudge an encounter is be more generous with Bennies if things are gong badly and less generous if they're going too well. Bennies work well as a subtle kind of difficulty switch.
Since I was asked, I thought I'd update my old recommendation above...
Until you become familiar with the way in which Savage Worlds functions as a system, it may be tricky to detemine if an encounter is appropriate to the abilities of the characters. To get a rough estimation, you can try getting a Damage Rating (DR) for your PCs. This rating is then compared to the encounter to give a general idea of how challenging an encounter may be.
Keep in mind though, no combat is guaranteed in SW; there is always a risk (as there should be in any dangerous situation) no matter what the capability of the opponent. Remember your "ace in the hole," bennies; the GM can adjust the flow of any encounter by the rate with which bennies are handed out to the players. To continue...
To figure the DR, take half the character’s "standard" damage dice (adding in any bonuses). For melee this would be half their typical weapon (i.e., the one they use the most) die and Strength die (a d10 Strength characer who usually uses a d8 longsword would have a 5+4=9). For ranged weapons, it would just be half the damage dice for the weapon (a 2d8+1 assault rifle would have a 4+4+1=9).
For variable damages, it's typically best to go with the maximum. A bolt can do 2d6 or 3d6 damage and a shotgun can vary from 1d6-3d6; in both cases, the DR would be figured off the maximum 3d6 value.
Then the GM can average individual DRs to figure out a DR for the group as a whole, comparing the overall DR to their intended opponent’s Toughness:
• If it's equal, then the characters have an edge over an equal number of opponents.
• If the Toughness is a point higher, then the fight should be about "even."
• If the Toughness is two points higher, then it will be a tough fight (One opponent for every two characters).
• If it's three points higher, the characters are in trouble (One Opponent for every four characters).
For purposes of this comparison, the opponents are figured to be Wild Cards as well, and for Extras, it should be considered that two are about equal one Wild Card.
Of course, there are other factors to consider, such as arcane powers, Edges, and Hindrances, but the above method should give you a rough idea of how your players will fare against the things you decide to throw them up against long enough to get a feel for the system.