How do you know which bond is the least ionic character?

It is based on electronegativity(EN) and atoms with nearly same EN or less difference in EN will have less ionic character because there is less charge separation.
C-H has very less difference in EN and is least ionic in nature.

Check this video on EN
https://www.youtube.com/channel/UC7Qv4kMOHwMLmH2Vk69aAig

thank you. I understand now, I watched the video.