To be aware of the indicators that signal the opportunity to change from an adjustable to fixed rate mortgage, one needs to study both the economic performance, and the cost of borrowing (interest) trends for the country.
The main economic indicator is that of inflation. When inflation is low, interest rates attached to the cost of borrowing tend to be the same. If inflation rises beyond what the government considers to be acceptable levels, then the treasury may use increases in the interest rates to bring this back under control.
As the interest rates rise, mortgage lenders will start to increase their lending rates. Usually their increase will exceed that of the treasury. For example, if the treasury increases by .25 of a percent, a mortgage company might put their rates up by .35 of a percent.
Other indicators, such as the reports from market experts, will give one some idea of whether the upward trend of interest rates is likely to be a short or long-term situation.
Therefore, to ascertain the right time to switch from adjustable to fixed rates is when a) the indicators suggest that rises in interest rates are considered likely to occur and b) when the markets suggest it is likely to be a long term situation, that the rates are likely to be high or continue to rise, for more than a few months.