You benchmark the system several times, each time with different settings.  For example, bench1 on stock settings as a baseline reference, bench2 with min cclock and max memclock to assess eth/xmr/other memory-intensive Lagos, bench3 with high cclock to assess gpu-intensive algos, etc.  It takes some time, but once you've thoroughly evaluated the system you have a lot of useful info.

Whichever algo your rig is best at, you mine at e.g. miningpoolhub, and autoexchange to the coin you want.