Was wondering what signs people look out for to indicate whether independent mining or pooled mining can become less feasible? and also, How can users detect problematic levels of miner pooling?
The level of pooled mining is never problematic.
If all miners are mining in pools there is no issue, as long as they are not mining in the same pool. What actually is a problem is if the majority of hashpower is controlled by a single entity. This is widely discussed on this site; search for "51% attack".
Feasibility of solo mining is not affected by the level of pooled mining. Most people consider solo mining infeasible due to the fact that they have a very small share of the global hashpower, which means they have extremely high variance, turning mining into a lottery for them. But whether most hashpower in the world is in pools does not affect you as a solo miner.