How will we know if the universe stops expanding?
The rate of expansion of the universe would begin to decrease. However the rate of the expansion of the universe is increasing.
In 1998 a study was done of quasars to check the hypothesis that the rate of expansion of the universe should be decreasing. This hypothesis was consistent with the prevailing view that matter and energy are eternal. The decreasing rate of expansion would be consistent with the theory that the universe recycles. That are there are alternating big bangs and big crushes where the universe is pulled back together.
The results of the study found that instead of the rate of expansion slowing down the rate of expansion was increasing. The empirical scientific evidence is that the rate of the expansion of the universe will continue to increase and will never slow down. This means that our observable universe will not recycle and therefor matter and energy are not eternal.