How is monoclonality or polyclonality determined?
There are a few ways to determine clonality. Sequencing-based methods are probably the cheapest, given the current costs of either sequencing, or the even cheaper PCR-based clonality assay. Other methods might include histomorphology, looking for known chromosomal aberrations and/or immunophenotyping, among others. Here's a nice review of some of the clonality testing methods. 

Sequencing and PCR would be the most definitive ways to determine clonality of a tumor, since they provide information on the exact sequences of sets of cells. 

Exome sequencing would be a good way to determine clonality in a large group of cells, where you don't necessarily know what to look for a priori. PCR is a lot cheaper and more focused and might make a better choice when you do have a specific target to sequence, such as highly polymorphic genes like some androgen receptors and the genes involved in VDJ recombination.