First of all you need to ask the which system they use or which school of feng shui are they from. All feng shui originated from Yi Jing from ancient times.
If they cant even tell you which school or system they use, you need to becareful.
One of the more reliable school of feng shui is "Xuan Kong" which includes "flying stars", this system includes time factors in the fengshui remedy.
Some school of thoughts does not include time as a factor, common sense will tell you that no one will have good luck and bad luck all the time. As time changes luck changes too. Thefore when the house is build or when was the last major renovation is an important factor.
with this information a chart can be plotted a good Feng Shui master will be able to predict pass events that happened in your house accuractely based on the chart. if they cant even tell you the past events accurately. how will they know what will come next and give you remedy?
One more thing you need to check is does the feng shui master recommend clients to buy a lot of fengshui products. In ancient times many of this products does not exist. Many of these products are not necessary, but "feng shui master" can make lotsa money selling these products.
These are some consideration before finding a good feng shui master.
