What do you call these cones used to mark a field for sports?
Calling them cones is the most obvious choice, "place the cones", "mark the field with cones" etc.
You might also refer to them as markers or marks.
They also could be named according to the sport they are used in e.g. a goal might be marked out with two red cones - you would likely refer to the pair as a goal as the usage of cones is already obvious.
These are normally called field markers, or marking disc in general language.