According to the tutorials the anticloak works by sending out a ping that bounces of an enemys hull (like sonar.
)
Check out the Bridgeitems.gf file in assets\commonsettings, it will tell you all the specs for the cloaking devices and computers.
If I read it correctly L5 cloak has a rating of 100, droping by 10 for each level. Rom cloaks are harder to damage.
L5 computers have an anticloak of 100, Fed/Rom/Klink L4 have a rating of 60, Borg 70, and Borg L3 computer 60.
Probes have a 10%? chance of detecting cloaked ships, if fired towards the location of the enemy.
Like bigcheese said ship size and ops officer skill also effects cloak and anticloak ratings.