In the opening, Poldark is asked the rhetorical question "Isn't that what you enlisted for?" Officers did not enlist, but were granted commissions.
We meet often Captain Poldark and other men chopping wood on nice grassy plain next to Atlantic scenery. They are far from house and with them is only one little log of wood and chopped wood is nowhere to be seen.
The series spans a time frame of over 20 years. Very early in the series, Demelza and Caroline have adult dogs. These dogs are present throughout the series and would therefore be well over 20 years old.
Ross Poldark is supposed to be a British Army officer, but he wears the uniforms of a Marine.
Ross shows up at the wedding with beard stubble. A man of his station would have shaved.