It's no different than casting a spell on any other person. Just because they are a necromancer - diviner of the dead - does not necessarily make it any more difficult to cast upon them with or without their knowing; definitely doesn't make it an impossibility. As with any spell you cast on a person only time and you will tell if they come to the realization of having been cast on - whether it be they find out by some divine or, feeling, intuitive means, or if you tell them.
Friend if the person is indeed good necromancer ,energy worker he or she will see it coming .Most advanced practicioners anyway work with number of spirit guides and of course necromancers work with deceased ones .Necromancers or Mediums uselly are psyhic anyway .Deceased ones despite being discarnated human spirits have the ability as every spiritual being to see and sense energy moving around .So they may as well inform him/her what is happening in case he or she did not notice .
Generally i agree with Vanity you can cast on advanced practicioners (necromancers included ) ,but how effective that will be depends of their skills to divine or sense it coming in some way either on thier own and either through their guides or spirits co-workers .The more advanced and sensitive the person ,the more spiritual beings co-workers to guide and protect him the harder will be for the energy you manifes to pass undetected and arrive the target .
In short possibility will depend strongly on their skills and the skills of their guardians (guides ).It will be possible for less experienced and less skilled practicioners and less probable for advanced ones .
You can drain yourself of vitalized etheric matter that builds up
behind your double. If you run out of it you'll have a hard time
utilizing psychic powers, but it usually doesn't kill you.
If the person is conjuring the dead too much, acquire objects
of deceased people or meditate
on the obituaries and let the images go through your mind
when your around them.