Sometimes I have not heard SO-50 on late night passes. Then I key up my transmitter with a 74.4 hz tone. After that, I key my transmitter on and off with a 67hz tone while tuning the receiver set in cw mode until I find a carrier that goes on and off with my keying somewhere near where I expect the downlink to be. I then give my station ID and change the receive mode to fm. I know that SO-50 is alive and well.
I don't know if it is well understood, but SO-50's downlink will only be on as long as there is a signal in the uplink with the 67hz tone on. You let up on the mike and the downlink ceases until someone else transmits. AO27 and AO51 have continuous carriers when turned on so they are easier to locate when there is little activity. (Little activity doesn't happen often here in North America on AO27 and AO51).
73, Dave W8IJ