Almost all mint straw compost is distributed locally. If possible, ask the source what herbicides have been used on it. Generally speaking, mint is not considered a food crop for humans, and so legally can have been sprayed with all kinds of things. I am not personally aware of incidents indicating the types of herbicides (clopyralid and chemically related materials) that survive the composting process, nor do I have any way of methodically tracking such incidents. It's always a good idea to ask your supplier. If the supplier is not the grower, he or she might not know. The direct answer to your question is: you'll have to decide for yourself. WSU offers a bioassay that would permit you to test any compost that you buy: http://puyallup.wsu.edu/soilmgmt/pubs/clobioassay.pdf.