Sorry guys for not mentioning what device im working with.
It is a simple smoke detector,
I played a little more after i posted the original post and found new solution, instead of getting 5v (from all pins)
i found only one pin gives around 2 +-.5v.
The pin im working with now is the green marked to most right (seems to be +).
The middle one seems to be ground, so when measuring the middle and right pins on alarm, i get round 2v
So im connecting the right pin to arduino analog pin 0 and reading its values, but when alarm has not triggered the voltage is 0 and arduino should be giving me 0 in output.
But im not getting 0 
Worth to mention is that when alarm goes off, it is not constant value i get, since the alarm is programmed to peep in delays. ( a peep every second )
no alarm: values between 0-1023 in random order
alarm: random nr - 1023 - random nr - 1023 - random nr -1023
Alarm seems to have a pattern that i can work with
but the problem is while no alarm, there is no pattern, im not able to detect if alarm has been set off or not :S
(Sorry for the bad explanations, electronics is whole new area for me)
EDIT: looking into fritzing now, looks like great tool for diagrams!