To monitor the Pi's temperature, look at: /sys/class/thermal/thermal_zone0/temp

To monitor the Pi's current frequency, look at: /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq

To monitor the Pi's PSU voltage, you'll need a multimeter, across the power supply test points, or the expansion header.

It's generally a good idea to keep the core temp below 70 degrees, and the voltage above 4.8V. (Note that some, not necessarily cheap, USB power supplies fall as low as 4.2V, this is because they are usually designed to charge a 3.7V LiPo battery, rather than to supply a solid 5V to a computer). Also, a heatsink can be helpful, especially if the Pi is to be run inside a case. A suitable heatsink is the self-adhesive BGA (ball-grid-array) 14x14x10 mm heatsink, part 674-4756 from RS Components.