A bidbot is a steem account that votes only on the post of the users who pay for It through Steem or SBD. Bidbots receive payments with a post link to upvote on posts. The post link is included in the transaction as a memo.
To recognize whether someone used a bidbot, you need to check his transaction history and the votes he received. A bidbot will always leave a vote on the post you want to analyze.
Many bidbots add a comment as well. If you check the comments and votes of the post, you will know which bidbots were used on that post.
You can get a list of active bidbots from https://steembottracker.com
@Tania12, In my opinion bidbot is an promotional service where one individual sends either particular amount of Steem/SBD to bidbot account and then bidbot account curates (Upvote) that particular post whose link we've send to bidbot account while sending Steem/SBD.
And these Bidbots are also called as Promotional Bots because, Bid Bot is nothing but an way of promotion of your product (Content Of Blog Post), and when we receive upvote then it will going to push our content to Hot/Trending pages depending upon the amount one individual sent.
In my opinion, there are many popular Bidbots whom we can easily recognise, but most commonly we can identify in comments section because if we see there are number of Bidbots which leaves comments after giving upvote and we can identify then.