Telltale signs are typically after you've played for a while, but eventually you can feel the weight of a string on your block. Some are still deceptive, mind.
Okay let's stick to the 123 example. Let's say it's a jab, axe kick, backflip kick.
1 (sM, ST) i8, r15, h20, -20b. -> This means 1 is a standing mid stagger, takes 8 frames for the fist to hit you, 15 for the arm to be retracted and be able to perform any other move to follow, gives you a 20 frame advantage on hit (the enemy will recoil for 20 frames), and is -20 on block so very unsafe.
2 (sOH) i15, r4, h40, 0b, -> This means 2 is an overhead so can only be blocked standing. Takes 15 frames for the axe kick to come down at the opponent, 4 frames for the foot to be back on the ground so that you can perform the next move, the enemy recoils for a whoppin' 40 frames if hit by it, and is 0 on block so safe to end the string with.
3 (sL, LA) i2, r30, h60, -30b -> Standing Low, LAunches, you get the rest of the picture.
Now you can check if that string cancels after 1.
Let's say we also have an armoured special to go with it, that's a headbutt which leads into a hard knockdown. Basically, the guy gets headbutted and faceplants. This hits overhead. Let's call it HB for Headbutt.
HB (SPEC, OH [~ex A]) i3, r70, h80, -8b.
And you are playing the same character whose d1 is i6, r8, h12, 0b.
Let's do work. Attacker performs 1,23
You can d1 where?
After 1, but not after 2 because not only are you blocking standing but the guy is even though neutral on block, is about to hit you with an i2 low launcher or an i3 overhead. You have to guess whether you'll block low or high, or you can risk armouring out after the 3 because if he doesn't realise that landing the 3 on a block is unsafe, the risk-reward is tipped against him.
Hope this sheds some light.
You should really go to youtube and watch the street fighter tutorial videos on fighting game fundamentals though.
