Whenever a paternity case takes place, a parenting evaluation almost always occurs as well.
Establishing parentage and custody battles can be difficult things to go through, and parenting evaluations are supposed to be for the benefit of the children.
Basically, in this situation, a third party is used to talk with everyone who is involved in the case and report their findings to the court.
This person can be either court appointed or the parents can agree on one.
  When it comes to your rights as a father, you want to make sure the evaluator is not going to be biased in favor of the mother, but will look at the case fairly.
If you can, use an evaluator with Master of Social Work credentials--that can be a good choice, and they are usually not expensive.
Many times a Guardian Ad Litem (GAL) is used for parenting evaluations.
  When this happens, not only are they doing the typical investigation work - talking to the kids, investigating references, etc - they are also the legal representation for the children.
  A GAL is used more often than not, and you may even be told it is required, but that may not truly be the case.
And unless abuse is involved, or it is a particularly ugly battle, a GAL is an unneeded expense.
If money is extremely tight, a court appointed social advocate (CASA) can do the job but you need to be aware they are volunteers.
They may lack the training to really stand for your rights as a father.
 Family Court Services are along the lines of CASA, but they are usually better trained.
  If your rights as a Father are your utmost concern, try to go with the best evaluator you can afford.