That's a very good question! In all honesty, it is hard to tell just reading a single post. I've upvoted posts in the past only to have someone ping me on steemit.chat to tell me it was plagiarized or worse. As a minnow, your vote doesn't really affect someone's reputation or payout very much. So I say vote away until you power up and get the hang of it!
That's also why I don't do a lot of manual voting. I have a bot that follows a group of curators. I don't want my voting power to go to waste. They've been pretty good at weeding out the PLG and Scams.
When voting manually I vote mostly for those users I follow. If I find they are not posting OC I quit upvoting and sometimes quit following.
Thanks! That was very helpful info.