We already have a list of how to identify the blind-bagged Transformers Kre-Ons, so it's only logical that we do their GI Joe counterparts.
There's a number printed along the right edge of the back of the bag, and the final two numbers in the sequence tell you who's in there:
...75 Alley Viper
...76 Cobra Ninja Viper
...77 Cobra Trooper
...78 Crimson Guard
...79 Cobra Axe Ninja (a.k.a. Dice)
...80 Jinx
...81 Kamakura
...82 Quick Kick
...83 Rock-N-Roll
...84 Scarlett
...85 Adventure Team Commander
...86 Torpedo
Now Series 2 is out, so here's the next set of numbers:
...58 B.A.T.
...78 Land Adventurer
...79 Mutt & Junkyard
...80 Flash
...81 Tunnel Rat
...82 Snow Job
...83 Blowtorch
...84 Slice
...85 Ripper
...86 Eel
...87 Dr. Mindbender
...88 Major Bludd
Series 3 has greenish bags with a red stripe along the top:
...03 Snow Serpent
...04 Tomax
...05 Torch
...43 Night Creeper
...44 Nunchuck
...50 Atomic Man
...51 Doc
...52 Cobra Air Trooper
...53 Footloose
...54 Lady Jaye
...66 Scrap-Iron
...86 Low-Light
Series 4 keeps the green bags, and tops them with a blue stripe. Unfortunately, there are no longer visible codes on the bags, so you will have to feel them up, just like you do Legos:
Air Adventurer
Airborne
Budo
Alpine
Chuckles
Cobra Shock Trooper
Cobra Viper
Nemesis Enforcer
Night Viper
Tripwire
Wet Suit
Zartan
Series 5 is green with purple. And again, no codes. Good luck:
B.A.T. (Battle-Damaged)
Bulletman
Cobra Driver
Dialtone
Iceberg
Leatherneck
Lifeline
Outback
Ninja-Ku
Techno-Viper
Tele-Viper
Zarana
Now you know. And knowing is something something something.
I found Quick Kick using this method, so I can at least vouch for him...
TRU finally had Series 3 today! Of course, I'd completely forgotten what all the numbers were, so I had to take my chances...