Look at the outcome of your structure analysis and the coloring pattern, if the colours form a distinct boundaries then no mixing, but if the coloring pattern looks like genomic data analysis through gplot, then, the lines are highly mixed and thus difficult to clearly group them to distinct population/ groups.

Thanks @Richard Odongo Magwanga

Use can use a online tool " Structure Harvester" link: http://taylor0.biology.ucla.edu/structureHarvester/

very easy to use.