You can mostly tell a bot by its name, most bots have fairly "proper" names (Olga E. Custodio, Melville Blanco, Leto Elo, Tam Stalsky, etc.); you can go into Proving Grounds to see bot names, or just kinda learn by playing (though be warned, there are some players using similar names to bots to throw people off). If you think someone might be a bot, check their rank on the scoreboard; three horizontal lines and a "proper" name (proper capitalisation, no numbers, etc.) means it's probably a bot.
"Captain Kill", if I recall correctly, is the kill credit for dealing at least 50% of the enemy's health in damage before they die, but not getting the final blow. It still counts as a kill as far as scoring goes (which is why, in games that are stopped at 20 kills, the actual sum of "kills" on one side of the scoreboard can greatly exceed 20).