It's easier to identify Chopin, since he developed a very personal style. For example, if there is strong chromaticism, it's probably Chopin. But it should be noted that, toward the end of his life, Beethoven too started composing in a more chromatic manner. Listen to the Adagio of op. 106 or the Arioso in op. 110. It doesn't seem like Beethoven at all: it's Chopin!
Other characteristics of Chopin are a strong emphasis on the melodic line, a rich musical texture, a tendency toward virtuosism and a very fast harmonic rhythm (but there are many exceptions to this one).
It's harder to identify a composition by Beethoven, mainly because he approached composing in a totally different manner than Chopin. Musicologists talk about the infamous "three periods", and such a categorization may work at times, but the truth is that Beethoven tried something new with every composition. He composed the 5th and 6th symphonies at the same time, and these two works are largely different for content and form.
Before 1813 circa, he rarely used contrapuntal devices. After that year, more than half of his compositions contains a fugue.
I think the only constant characteristic he always had was his obsession with motivic development. If the theme gets constantly developed in new and different ways, it may be Beethoven.