Alternative testing techniques

In special education we learn to get creative with how we deliver our teaching and test our students. I have a group of students in my class who i have struggled to formally test. I have used many different techniques to find out if they know their alphabet. For a teacher who has a student who is non-verbal it can be challenging to find out what they know.
Usually i would get them to choose out of 3 cards "show me ___" however what if they cannot yet make choices or don't understand the task? What if they cannot respond to simple instructions like "point to", "show me" or "give me"?
I have a student who i have attempted to test 8 times, ive tried everything from building alphabet towers and getting them to find the correct alphabet block to using  letter stencils to make letters in play dough.
Benjamin (part of my focus group) works well with technology however i couldn't find an app or game that was suitable. I have access to a smart board in my classroom so i made a game where Banjamin had to pop the balloons with the letters on them.
I used peer modelling initially to teach him the task. I used a student who had good alphabet knowledge and as i read out the letter, he would pop the balloon. When it was his turn he completed the task correctly, i was astounded that he knew all his lower case alphabet. I tested him twice to ensure that my data was correct and managed to capture some evidence on video to show his family.
Now that i have found a method to test him, i look forward to finding out what else he knows.