In my state (US) there are three ways to approach this problem.
1) Pick the most obvious images and charge based on those. (most common, typically prosecutor limits us to only a handful of images anyway as each image is a separate charge here)
2) Have an expert identify the victims in the image. (in my jurisdiction its a Sexual Assault Nurse Examiner who is a CNP who specializes in these type of cases)
3) Let the jury decide (which usually happens regardless of other two)
Personally I will only call the SANE to look at not-so-obvious images and save the clear cut one's for the prosecutor. They will then pick the best images to use for trial.
EDIT
We also can send the hashes of the images to the National Center for Missing and Exploited Children and those may return a hit for a known victim. There is also the Child Victim Identification Program which can help identify known victims. If any of those hit then its easy.