Determine if YouTube video is widescreen?

If you're open to using a different method other than V3 of the API, then I believe it is possible via the oEmbed API.

http://www.youtube.com/oembed?url={VIDEO_URL}&format=json

Like so:

http://www.youtube.com/oembed?url=https://www.youtube.com/watch?v=zMJ-Dl4eJu8&format=json

Would produce:

{  
    "provider_url":"https:\/\/www.youtube.com\/",
    "thumbnail_url":"https:\/\/i.ytimg.com\/vi\/zMJ-Dl4eJu8\/hqdefault.jpg",
    "thumbnail_height":360,
    "height":344,
    "type":"video",
    "version":"1.0",
    "html":"\u003ciframe width=\"459\" height=\"344\" src=\"https:\/\/www.youtube.com\/embed\/zMJ-Dl4eJu8?feature=oembed\" frameborder=\"0\" allowfullscreen\u003e\u003c\/iframe\u003e",
    "author_name":"hadronica2",
    "width":459,
    "provider_name":"YouTube",
    "author_url":"https:\/\/www.youtube.com\/user\/hadronica2",
    "title":"Aikido - Kazuo Chiba sensei - 1\u00ba part",
    "thumbnail_width":480
}

In the examples you've given, the output was as follows:

http://www.youtube.com/oembed?url=https://www.youtube.com/watch?v=zMJ-Dl4eJu8&format=json

Width: 459
Height: 344
Ratio: w/h = 1.3343 = 4:3 (ish)

http://www.youtube.com/oembed?url=https://www.youtube.com/watch?v=zMJ-Dl4eJu8&format=json

Width: 480
Height: 270
Ratio: w/h = 1.7777 = 16/9

This appears to work in the examples you've provided.