use linear regression slope as an indication of trend set minimums for both up trends and downtrends, with mid trend equating to the area between both minimums
For a trade indicator to be useful I believe it should be consistent and not signal a trend if the underlying price generating process does not include one. The problem is that even if you construct a time series with no trend component, an increase in volatility may make some trend indicators signal that a trend is present. The paper discussed earlier provides some examples. Or you could check it out in Excel. Another important aspect is that the result provided by the indicator should correspond to what you see on the screen.