OK, why limit it to a good night for DX? Why not a good day for DX? There is a DX world above 10 MHz that is pretty active during daylight. And different signals are going to be used as indicators for different regions and different times. For example about 0700 local time I will have a good path to Asia/Pacific on freqs below 15 MHz, and a good path to Europe/Mid East/Africa on freqs above 12 Mhz. So I will have to look at different signals as indicators of each path.
After you get used to the conditions, noise levels, and normal signals in your area it becomes pretty obvious pretty quick what the conditions are after tuning around for just a few seconds.
For me I tend to look at some of the “steady” signals as indicators. If I can see the Chinese OTHRs in the 4-8 MHz range I look at the specific levels. If levels are good I know conditions will be open to Asia. I look for the Akrotiri/PLUTO radar out of Cyprus, it tends to follow the MUF pretty well and is an indicator of an existing path to Europe/Mediterranean/North Africa. Looking at where I see CODAR radars can tell me what bands are hot to coastal regions. I say “see” because I do a lot of my initial evaluation of conditions visually, using a wideband display on an SDR. With a 0-30 MHz display you can see at one glance what bands are active and what are not.
The ham bands. These are distributed across the HF region pretty well and give a pretty decent indicator of what is happening propagation wise.
I look for VOLMET stations world-wide (
SW Volmet Broadcasts ), they are relatively low power stations, widely distributed, and pretty easy to ID, if you hear one from a target location you can be sure conditions to that area are open. Although I do not have a good single resource to link you to I also use the Maritime HF Weather reports (both voice and data, you don’t have to have a decoder for the data to use it as a propagation indicator) from stations around the world.
Time stations (time and frequency standards) from around the world are a natural also. WWV is pretty much an all the time thing here, so I try to ignore that, but WWVH can indicate things are bubbling to the west, on whatever freq I can hear it. Many other nations have time stations on the same frequencies as WWV/WWVH and sometimes a little digging there will indicate a path also, BPM out of China and YVTO out of Venezuela, for example. RWM out of Russia on 4996, 9996, and 14996 are nicely clear of other time stations. CHU Canada can be of some help, but even under poor conditions I can most often hear that, I guess the lack of CHU is an indicator for me. HD2IOA out of Ecuador on 3810 and 7600 are also on unique frequencies. If you are going to use time signals as indicators be aware not all stations are on all the time, and each might have unique features that help to identify what you are hearing, a good basic resource with descriptions and schedules is here:
http://digilander.libero.it/occultazioni/segnali-di-tempo.pdf
T!