No, you can't. But normally you can read out the temperatures of the important components (CPU, GPU, RAM) with one of many freeware tools. Temp in the case is usually not that important :).
Again, you would need software to do this, as I wrote in your other question, I would do it via the mainboard and then monitor it with HWinfo64 or something similar.
No, the PC will certainly not overheat because the fans turn too slowly, the CPU fan is controlled by the mainboard anyway. Is there a reason why you want to install a manual control? To read the RPM you would need a software, either from the mainboard manufacturer or for example HWinfo64. I would run all fans through the mainboard and adjust the fan curves manually. Such fan controls are oldschool, I would say.