AdMob Check if video ad has finished then award currency

Hi everyone,

Im sure this quite a standard thing in game ads but I've not done it before and can't find any info.

I want to allows players the option to watch a video for X currency. How would you check if the user has watched the whole video and award them something if they have?

Thanks

 

any one knows how i can know if the user has watched the video or skip it ! !