How do old folks determine weather conditions?
Well, I think "old folks" probably determine weather conditions the same way everyone else does, which is by watching the evening news.
If you are talking about weather determination from ages ago or folklore here are a few different ways that people determined what the the weather would be like.
There used to be Seagull forecasts. Seagulls like to be in flight and they like the water. They even prefer to sleep on the water. If seagulls were perched on land it gave people an indication that bad weather was already present or very near.
People also determine weather simply by observation (as we still do). If the skies are very clear, especially to the west, it usually meant that bad weather was not in the forecast. Occasionally in meant that a bad storm was approaching and the skies were clear due to an updraft from the bad storm. Fog, moisture, and clouds are all indicators of weather. Cows, for example, do not like to stand with their faces in the wind so they stand with their backsides to the wind. This is one way that farmers knew which direction the wind was blowing in.
From personal experience, my grandfather always new when rain was approaching by joint pain. This was caused by humidity (moisture) and is often felt by people of varying age groups, not just older people.