Technically you could put the bull in at the same time as the embryo transfer, but if you aren't going to be able to tell whether the calf is ET or natural by looking at it (i.e. it's gonna be black either way) you could wait 2 weeks to put the bull in, then preg check during the ~2 week window when it has been at least 28 days since the embryo was transferred but it has been less than 26 days since the bull was put in. If you want to wait longer than that to preg check then I would wait 30 days so you can better distinguish between the age of the potential pregnancy.
DNA Parentage tests are always an option too