Measure laser with photoresistor (LDR)?
Is it a good idea to use an LDR or should I use something else?


I'd go with something more sensitive, a photodiode or phototransistor.


  I saw these photodiodes, but they can't determine how much it is, just if or if not.


That's simply wrong. Just because you haven't calibrated them doesn't mean they wouldn't be the better measurement device!

In fact, CCD camera sensors work based on the principle that you bias a diode reversely, employing the depletion zone as capacitor. Incident photons lead to charge migration (i.e. current), which leads to a readable voltage change across said capacitor. So, as CCD sensors are typically these that you use on the more demanding light measurement settings: can't agree.

From a personal experience: red laser pointer, pointed at red LED with a Darlington array amplifying the photocurrent works pretty well as a demonstrator.