By reading the Twitter terms of service.

"You retain your rights to any Content you submit, post or display on or through the Services. What’s yours is yours — you own your Content (and your incorporated audio, photos and videos are considered part of the Content)."

(Twitter certainly isn't unique here, this is the gold standard for user-submitted content. Users always own their content, but grant the service an unlimited royalty-free transferable license to copy, process, display and "perform" the content.)