White spots on retainer?

[Jamie H]

I have had my clear plastic retainer now for around 4/5 months I keep them in all day take them out when eating and wash/brush any saliva off each time. I do clean them with tablets and mouthwash on a regular basis and haven't had any problems till now as I am starting to see a few white spots appear on the clear retainer that won t come off with brushing or tablets. How can I tell if this is plaque or tartar or just pressure marks from biting down with usage? It only seems to be on bottom retainer