How to know if 'Unguarded Nomadic Frontier' event already fired?

Unguarded Nomadic Frontier is a disaster, not an event.

If you are China, you can see whether a disaster is brewing or not on the stability & expansion tab. Disasters are at the top next to your war exhaustion.

The requirements for unguarded nomadic frontier are the following:

• Country must be Emperor of China
• does not have an ongoing disaster.
• has a neighboring country that:
  • has the Steppe Nomads government reform.
  • has a total development (including non-tributary subjects) of at least 300.
  • is not a subject.
  • does not have a truce with the Emperor of China.
  • does not have an alliance with the Emperor of China.

If you aren't China, you have to manually check whether the above conditions are fulfilled and the disaster progress starts. If the disaster happens, you will get a popup notification. The disaster can only happen once, so if you missed it, there is no way to know.

If you are the horde next to Ming and have the appropiate governemnt reform, the disaster should happen after around 8 years of establishing a border, unless another disaster is running in China.