Hypothesis testing : Constant mean over time for two conditions within an experiment

What is an approach to testing/showing that within a particular period of time, there is no change in values. So, I have my response variable measured over 30 seconds, in two conditions. I want to show that in the last 10 seconds my response variable is constant in both conditions A and B. My response variable has one value per condition.

This is a between-subject design so one subject participates in one condition.

Sub Trial Meas.
1     1.   2
;
:
1.    30.  7
2.    1.   2
3.    2.   4
:
:
3      30.  6
4.     1.
:
4.    30.  4
5.         2