Switching transformer parameters

Hi

I am planing to design forward off-line converter using UC3842 with the following ratings:
Input: 100 -240v
outputs:
12v 3A
5V 250MA

The problem is that the transformers available in the local market don't have any listed specifications ( Core material ...etc) they are E type ferrite this is all I know so is it possible to do the design without knowing the transformer specifications for example by trying different frequency ranges and else.

Also I have few damaged ATX power supplies so can we predict the specifications of its transformer I mean do they share common specifications?

Thanks in advance
Alexan

Any clues?