There is small difference in power consumption when you compare a phone with laptops.
Laptops most of the times runs on direct power through charger. (Mostly we carry charge with laptop).
Mobiles mostly run on battery power. (Rarely we carry charger with us).
By keeping this in mind, android has designed in a such a way that, phone will not consume battery power if user is not interacting with the phone. This is to increase battery back up time for user.
That's why when LCD screen power is off (that means user is not interacting with phone), with in fraction of seconds CPU will also go to a mode (sleep mode) where it does minimal work and saves battery power. When CPU is in sleep mode, it accepts commands only from RIL (radio interface layer) which is basically SMS and call functions, and alarms. Other than this CPU will stop executing other applications functions.
Note : If any application wants CPU time for its emergency work when CPU is in sleep mode, then it can request CPU time by using WAKE LOCKS.
For eg : MP3 application has to keep playing songs in its service, even though user has turned off LCD screen. That means MP3 application's service has requested CPU time by using WAKE LOCKS.
Your question : How do i know when my phone will go to sleep?
- Ans : Simple it goes sleep mode when LCD power is off, with in fraction of seconds.
Your question : Does sleep mode in particular device varies from brand device
- Ans : Since android is open source, any OEM can take driver layer , which is GPU license and can modify the power manager drive to behave differently. All these sleep modes and wake locks are finally under control of Power manager driver of Linux kernel layer.