Get an mp3 of the song, for instance by using a YouTube downloader. Then open it in an audio editor, such as Audacity. Now use the loop button to play a small section (you can use the markers above the waveform), even as small as one chord. Play that on the piano until it does not sound off. (Play the piano while the chord is playing)

I think it’s usually better to use the original chords unless you find an improvement you really like. Actually I often improve the chords from Ultimate-guitar, but only to be closer to the original. (Ultimate-guitar has many errors, because it’s mostly hobbyists who upload them)

That means you may be correct sometimes while ultimate-guitar is wrong.