When it comes to more formal or comprehensive assessments I typically use the SETT Framework to develop my assessment plan (the communication portion of the W.A.T.I. also has some fantastic information as well). If you are searching for a standardized assessment (and have access to one) the TASP is full of great information about symbol recognition and grammatical use, however, there are many other options available for informal assessment and progress monitoring.
(1) We have talked about this app before here but the AAC Genie app for iPad is an awesome tool for evaluating different aspects of language development and AAC use.
(2) Linguisystems puts out the Augmentative and Alternative Communication Profile which is a great way to quantify current skills and helps to identify targets.
(3) Lastly, a wide variety of AAC data sheets is crucial to monitoring progress and identifying strengths and potential areas of deficit.
Check out this post from PrAACtical AAC for more great recommendations.