Mainly just Googling and looking at benchmarks. You can't compare cards even between different generations, let alone different manufacturers, on core count and clock speeds alone, so benchmarks and reviews are the best way to compare.
Edit: I should add, gpureview.com although not always 100% accurate is a good way to look up certain things at a glance. The main thing which makes the MX440 faster is the 128 bit bus compared to the crippled 64 bit bus on the 9200SE. The MX440 is older though so as you can see, it only supports DX7 and OGL 1 vs DX8.1 and OGL 1.4 for the 9200SE.
