Use the is available block in the same scene as the initialize block. This tells you if the ads are available.
The show rewarded/video ad is automaticly check if ad can show. In your case if you want to ask the player first, you want have an extra block that says the ad canshow. (if ad cannot show it will returns false else it will always be true. This methode is for both ads the same, there is no seperate methode available in the unity SDK so i cannot make 2 blocks 1 for video and 1 for rewarded. I will update the extension with this extra block.
Please don't compere ads network with other ad networks, there SDK are different, with unity you don't have to cach or load, that methode is not available in unity SDK.
EDIT: I Need completely rebuild the blocks and methode to get this to work, and than it will be complicated to get the blocks the right way for the user with no knowledge of programming.
What i can do is implement an Alert dialog that ask the player if he want to watch the rewarded video or not when its ready to show the rewarded ad.