"How do you tell it is no longer fresh or effective?"
Cannabis flowers are similar to any other fresh produce; sunlight, temperature, humidity and O2 degradation wreak havoc once the flowers have been harvested.
Temperature tends to do the most damage, so try to keep your stored flowers in a container that never gets warmer than 80F. Many of the subtle, delicate aromas people love about fresh cannabis, are produced by a highly volatile group of essential oils called terpenes. The most volatile of these compounds are the monoterpenes, and they begin to evaporate out of flowers very rapidly once ambient temperatures have reached 80F. At PolyKulture Cannyard, we cold-cure all of our flowers at 68F-72F in order to retain as many terpenes as possible. It can be a bit overkill, but our farm has been known to keep our most treasured flowers in the refrigerator to retain every bit of freshness we can.
By keeping flowers in the fridge, while also mitigate degradation caused by UV exposure. ALWAYS keep cannabis out of the sun! it's probably fairly common sense, but if you leave a jar/bag/container of cannabis in the sun for a very short time, you will find that the combination of heat and UV has significantly degraded the flower to the point that it brown in color and not enjoyable to consume.
Cannabis flowers are generally brought to market at a fairly dry 10-15% moisture content, but depending on the environment they are stored in, moisture levels can fluctuate, so keeping your cannabis in a sealed container helps mitigate changes in moisture. In coastal areas consumers must be careful not to leave cannabis out and exposed to the atmosphere, because the dry cannabis will absorb moisture from the air making it damp and difficult to consume.
Lastly, the O2 molecules that make up a part of our atmosphere, are constantly breaking down all agricultural products. This breakdown is difficult to mitigate and can only be avoided by removing all O2 molecules and exchanging them for another type of gas, generally Nitrogen. This breakdown due to O2 exposure is the biggest contributing factor to the lack of flavor or taste in a pre-rolled joint, as opposed to a fresh roll.
In the end, if you treat your cannabis anywhere near as well as you do your vegetables, eggs and meat, your cannabis will do great!