This really is not a MATLAB question, as you have no idea how to compute what you want, and your question is too vague to have a true answer.
There are many ways to see if data has a trend over time. For example, you might do a simple linear regression fit, thus y as a function of time. Test to see if the slope is significantly different from zero. If the slope is bound away from zero, then you have an answer.
Or you might simply choose to do a split the data into two segments or blocks, as a function of time. Now do a test, comparing the mean of block1 to the mean of block2. Are they statistically different? You might use tests like an anova, or a non-parametric test. Lots of them in the stats toolbox, and I am not a statistician to say whch is best, even if I did play at it in a past lifetime.