Say a ceramic capacitor with the code 103 (0.01 micro farad). How to identify its voltage rating? Searching for a 0.01 micro farad capacitor in Mouser the voltage rating is indicated as 1kV dc. I intend to use the capacitor in a RF receiver circuit((20Mhz) where the maximum voltage is 12 V. Is it ok to use such a high voltage rated capacitor in my circuit? Also if I don't have a capacitor of a particular value,will resolving the value into series or parallel combinations affect the quality of the receiver?
Like other components, ceramic capacitors have a voltage rating. Looking up 'something of the same value' in a catalogue is not the ideal way to do things. It it's not marked, then keep it in labelled packet after you've bought it.
With 'low' values, like your 10nF for instance, if it's fairly large, a few mm long or so, then you can assume it's a low K ceramic, and like Richard says, is probably rated for 50v or more.
Unfortunately with higher values, and smaller case sizes, high K ceramics are used, to cram all the micro Farads into the tiny volume. Here you will find voltage ratings of 3v, 5v etc. One very interesting trait that you get for free with high K ceramics is a vicious voltage coefficient of capacitance. You can easily end up with half, yes 50%, of your specified capacitance if you use a high K at its rated voltage, which doesn't go down well with LDOs that might need a minimum C to stabilise them. Ask the manufacturer for a data sheet specific to the range, value, voltage, dielectric and case size of the part you are considering, as all of these can affect the voltco. The distributor shortform datasheet is usually not sufficient.
Building up exact values with series or parallel combinations of ceramics caps is generally OK. The problems you can run into are due to the residual inductance of either their length or the extra wire you use, causing resonances with the other capacitors. For instance, one common problem is caused by paralleling a 100nF and a 1nF (a much touted combination for decoupling). Unfortunately, the 100nF goes inductive at a frequecy where 1nF is still a good cap, and they go parallel resonant, high impedance, usually in the 100s of MHz.
How to identify its voltage rating?
If you don't know what the part is (i.e. the part number from a supplier) then you cannot rely on anything about it other than what is printed on the component and, if this doesn't contain a voltage rating then, the part becomes junk.