https://steembottracker.com Will help you with this. In the list of bots present on the site, check for a "detail" tab present on the right hand side. Click on it and new screen will open.

The new screen will look something like this.

Here in the above photo i have marked the place where it shows negative ROI. when the bot shows negative ROI you should not bid for it because you will lose money there. There are many who bid even after seeing negative ROI.

If the screen show green then that means these bots are giving positive ROI. You can bid on these bots but be careful only bid for the value that is remaining. Do your own calculation. If the bot has 5$ worth of vote left and you bid more than 2.5 sbd then that means you are going to go under loss. If you bid for 2 dollars then you will get positive ROI. If someone decides to bid after you then it is his choice and he might run you for a loss.

Negative ROI happens in the last 5 minutes mostly. Some people randomly bid based on luck with out doing calculations at all.

I hope this helps.