First off, if you are not a civil structural engineer yourself then Guru is right that you need to hire a professional Engineer to do this job.
If you are a structural engineer then you shall get a full structural survey carried out to establish the existing loads and the existing member sizes and details along with its foundations; then you need to analyse the tower and its foundation for its capacity; compare the existing capacity to its existing loads. Then you'd find out if its overloaded or any remaining capacity is left over to take any further loads. Hope this lelps else email me back.