Trustworthy of firmwares / mods

[Guest srynoname]

Hello,

I'm just wondering if there's any way to check the trustworthy of non-official firmwares / mods? And on what firmwares are they normally based on? E.g. lets take the ZTE Blade / Orange San Francisco - any chance to get a "checkable" firmware for that? On what are alternative firmwares normally based on? Any chance to take an official android 2.2 firmware image (where would you get this?) and use it with the ZTE Blade? Would you need to change deeper system internal things?

Shortly formulated, I'm interested in upgrading a (rootable) android device myself (includes also usage of alternative provided firmware images if you can check they are not tampered), while still making sure nothing gets tampered.

Thanks for any hint!