You can tell that they're ads because they're not from accounts you follow and nor retweeted, and you can verify that they are are ads because the three-dot menu includes the "Why this ad?" option.

“Per X, the MrBeast video is technically not an undisclosed ad. There is a pre-roll ad for Shopify in the video, which is labeled as such. X boosts posts containing pre-roll ads, but because the post itself is not the ad, it doesn't have the label.”