if you have Can Grow set on any cell, all bets are off...CR does not give any methods to interrogate a cell and see how tall it is (ie if word wrap has occurred).
Unfortunately, the most common method is to put a counter of some sort (I like shared variables) so that each line of detail counts. In the Section Expert, in page break before, x-2 button put something like:
shared numbervar lineCount;
lineCount > 25 //or whatever value you want
in the page header you would want something like:
shared numbervar lineCount :=0;
"" //hides the reset
in details
shared numbervar lineCount;
lineCount := lineCount + 1;
""//hides the increment