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lets say, someone gave me an android phone ( with custom rom, lets say CM or etx). i did hard reset to erase alll data and usr apps. but i am interested, how to know if that custom rom is not modified ( if i am afraid of spyware hard-coded into that os).
how to check if that system is clean from such custom modification, and is an original CM?
EDIT: I've found interesting answer here - How to tell if phone's ROM is original or tampered?
No real way to test it unless you know what you are looking for (in terms of spyware) the best solution is to flash a new CM rom from an approved site that is known to be clean.
You haven't been informative in your question.
Clean install with formatting or wiping of partitions is already suggested.
But, if new installation is out of options for some reason - ROM not available, can't install because you must use supplied platform or you actually want to discover whether something malicious is installed or not, you can do debugging and/or traffic sniffing from device.
I recommend Android Studio for debugging - Android device monitor.
