need a little advice on doll heads

Symmetry is pretty tricky a deal to grasp fully. I'd say it comes gradually to you
But there's some nifty tricks to help. Art-iculated/Sailor seems to use mirror to compare sides, if I remember right.
I stretch out proportion likes out of the middles/eyes/mouth and other significant points, like you would in a drawing of a face from front. However, that doesn't resolve problems with the organic planes so much.
Pictures are my other personal reference. For example, flipping pictures horizontally should help you pinpoint if something is off, and it's great to keep tabs on any loss of detail or features you accidentally modified in the process.
Hope this is helpful in some way! Do show your results so far

 

Thats a good start tho!
From what i collect symmetry wise, you'll just need to check the contour of the face and the cheeks as you define them
Adding depth to the eyelids/eyewells
In the case of FIMO i'm pretty sure I've seen people pin the eyes into the inside of the head to make it easier to shape them

 

One of the things I did when I was working on my doll head in clay was to use a clear plastic sheet (the lids from little plastic containers of nuts, in my case) to draw on in permanent marker, making outlines for one side of the face, and then I could flip it over to be able to check the symmetry fairly easily on the other side of the face. When I got to finer details, I would turn the head upside down so I was working on it that way, as it makes it easier for your eyes to see asymmetry when your brain isn't interpreting it as a face, but as shapes.

You can also use a mirror and hold your head up to that - asymmetries and problematic spots will jump out at you even if you couldn't see them un-mirrored. And yeah, flip photos horizontally, too!

Some of the most realistic dolls I've seen have almost been anatomy studies built from the inside out - so start with a rough skull shape, set the eyes in place, begin building the muscles, add fat and skin on top, and then add details.

 

That's how I sculpt too! What's important in that method is do not start sculpting mouth and nose until you have the "meat" under them symmetric. If the underlying surface is lopsided, your nose or mouth will seem unsymmetrical and wrongly placed no matter how perfect they are on their own.

 

I start at the middle. I make a flat plate of the doll with the nose and lips, and the back of the skull. Then I just add clay to make it horizontal.

 

one thing that helps me with symmetry is taking a number of pictures and examining those on a computer screen. this somehow makes it easier to spot imperfections.

 

Something that might help for both symmetry and keeping the clay from distorting while you work on details is to bake it in stages.

Make your core and fix your eyes to that. If you are going to open the head and make the eyes removable, make sure to imbed the eyes enough into the core that they don't get stuck in the clay. You can also use glass marbles or beads as place holders for the eyes.
Then you cover your core in clay. No details, no facial features yet, just cover it and make the basic head shape.
Make sure you have symmetry here, make the general shape of the head, mimic the shape of a skull if you are going for realism and most importantly, make sure the whole thing is a little smaller than you want your finished head all over. You can get some decent symmetry "for free" by adding equal sized balls of clay to both sides of the core. You can ever measure and draw guidelines on your core if you feel it help, scales to weigh the clay, rulers or measuring tapes at hand. Use the mirror trick, the clear plastic trick, the paper template trick or whatever works for you. Just keep at it until you are happy.
Then you bake it.
Don't get it too thick, polymer clays don't always bake evenly if the layers are too thick. If you are making a very big head, consider a larger core and perhaps cover it in two layers instead of one.

Once the covered core has cooled you add the "skin" layer.
Just put a big flattened ball of clay and push it down, being careful not to trap any air under it (this is important, air bubbles can expand when baking and distort your work) and smooth it out so that it is fairly even. Cut over the eyes so that the peak trough and then you are ready to start sculpting for real. The firm core, the fixed eyes and already having the general proportions down makes it much easier to get something face-shaped.
Personally I don't like additive sculpting in polymer clay, the risk of tiny bubbles is always there (but I work in super sculpey, which is slightly translucent, so the smallest flaw shows). I prefer to put the right amount of clay on right away and then just start shaping it. That way, if you put on equal amounts of clay on both sides, you get symmetry. All you have to do is shape it.
If you find that you have put on too much clay, just push it towards the back of the head, making sure, again, to do the same on both sides. When you are done you cut the surplus of, before shaping the back of the head.

I usually shape the face, bake it and then do the back of the head and bake again, but it is fully possible to do it all in one go.

In the end, you will simply have to try different things out and find what works for you.
Looking at anatomy books, books about figure drawing and such might be a good way to get familiar with the basic proportions of the human face.