Even pre-Google a lot of plagiarism was relatively easy to spot. Unless the entire essay is taken wholesale from one source (which means that the question was too generic), one can usually spot differences in the writing style where sentences and paragraphs have been cut-and-pasted – moreso these days, when students all seem to be incapable of writing a coherent sentence on their own.
If the class is small enough (<30) that you have been able to interact with each student, you can also get a sense of 1) their level of comprehension of the concepts involved, and 2) their vocabulary level. Papers that jar against the image you have of the student’s ability to communicate automatically get a closer look (which is not to say that someone can’t write more fluently than they speak, but it’s worth checking anyway).