How do I know...

[count123]

How do I know what tweaks, themes etc. I can safely flash to my phone?

 

[3cit]

How do I know what tweaks, themes etc. I can safely flash to my phone?

Former android user!!!

Anyways, if it's on a reputable cydia source, it's safe... Worse that happens is you'd phone resprings to safe mode and you have to uninstall new tweak

 

[Bias X]

Sometimes warnings will come up in the description if its incompatible.
That or just read the description, some of them include for iOS 3.x, 4.x, and or 5.x

And as 3cit said. Worst case scenario, you restore your phone. More than likely though you'll just spring into safe mode and be able to delete it.