What are the signs of necrosis after lip fillers? Would my practitioner be able to tell straight away, if they're experienced?

HI 
It is very rare for an experienced injector to encounter that but usually sever sudden pain occurs and either blanching or purple/black discoloration occur. Every injector must carry the dissolving agent handy in case this happens.  

Signs of Necrosis
1) Pain - might be difficult if Anaesthetic used
2) Blanching - area appears pale
3) Purple skin colour
4) Skin appears cold
If these are occuring the antidote is needed urgently