Is the flow of time regular?
Picture a leaf floating downstream on a river in the pitch dark. As the river gets narrow it speeds up, and as it gets wider it slows down. The leaf has no light to see the shoreline so from it's perspective there is no change in the flow of the river.
Now what if the light turned on?
The leaf could now see the shoreline, but from a physics point of view. This shoreline is the background radiation of the universe. It's so far away that as we move down the river we can't perceive any movement in the shoreline. Therefore, we can't tell if the river speeds up or down.
The only way would could tell is to look at the shape of the shoreline. Are there points on the shoreline moving closer and/or further apart. How could we tell the difference between changes in time or changes in the shape of the shoreline?
That is why people study the big bang. To predict what the shape of the shoreline should be, and then see if there are any unexplained changes.
How would we come to know if the whole universe along with everything in it stops for a while(may be a century)
Time for a human is perceived not in the current moment, but in the memory of previous moments. Humans remember time passing as our brains record past events. If time stopped for one individual, then it would be like pressing pause on a VHS recorder, but when time stops for the universe then everything stops. Even the VHS recorder would not record a gap in the video since the inputs to the VHS paused as well.
The universe could stop and continue. We'd have no perception of the events. Since it's a completely untested event. It means it never happened. Even if it did.