In designer view set your combo box's
DropDownStyleproperty to
DropDownList or by code:
myCombo.DropDownStyle = ComboBoxStyle.DropDownList;
You can check whether some value is selected or not by checking
SelectedIndex or
SelectedValue property, it depends on how do you fill data into your combo boxes.
if(myCombo.SelectedIndex != -1)
{
}
if(myCombo.SelectedValue != null)
{
}