People make reproductions using moulds taken from the original heads - the original heads have fairly deep incised markings and these are therefore transferred to the mould and hence to the new heads. A good artist will always sign (and often date) their work, usually next to the incised copied mark – check this first.
There are several ways of telling whether you have an original or a copy if it is not a signed reproduction.
First, nearly all antique heads are made of white bisque which is then completely tinted on the outside to resemble skin, newer heads are often made of tinted bisque, so if the inside of the head is not white (allow for dirt) then it is likely to be a copy.
Second, with experience you can tell by the painting that it is a copy - there are very few artists who can paint reproductions like the originals and they nearly always sign their work. Check on eBay for other examples of the same mould or same maker and look for differences; eyebrows are often a giveaway as they are very difficult to get right, the colour of the mouth, the colour of the skin, the cheek blush etc.
Thirdly, heads shrink when fired, so a reproduction head made from a mould from an orginal head will be smaller than the original - there are often size charts you can check.
I hope this has helped you – please contact me for further information and I’ll do what I can to help.