Guidelines for determining shock hazard of capacitors
First, it is not the capacitor that can harm you, but the voltage and charge stored in the capacitor. So all capacitors are safe when uncharged, which is what they are when you buy them.

To do harm to your body, the voltage across the capacitor's terminals must be high enough to cause a harmful effect on you. There are no hard rules for at what voltage things become harmful, but a common 'rule of thumb' is that DC up to 48 Volt is considered low voltage. So a capacitor charged to a voltage below 48 V is fairly safe. 

That does not mean that a capacitor that is rated for 25V is necessarily safe: it is guaranteed to work to 25V, but it is not guaranteed that it won't work up to let's say 70V. And it also does not mean that a capacitor that is rated for 1000V is harmful: it is only (potentially) so when charged above 48V.

There is another form of harm: a capacitor with a very large capacity, charged to an otherwise safe voltage, can cause a very high current when its terminals are shorted. The sparks and heat can harm you, and the capacitor itself could explode. No need to worry about this effect with you garden variety capacitor up to below let's say 1.000 uF, but shorting a capacitor is something you should avoid nevertheless.