Slovak and Czech languages are closely related and on cursory examination are very similar on the printed page. In twentieth-century texts it is possible, however, to make a positive identification of the languages. This is done by looking for specific letter-plus-diacritic combinations.
The letter U with a small circle above it - ů
This is typically at the end of a noun, and occurs only in Czech.
The letter E with a haček above it - ě
This occurs in the middle or at the end of a word, and occurs only in Czech.
The letter R with a haček above it - ř - occurs only in Czech
The letter A with an umlaut above it - ä - occurs only in Slovak.
The letter O with a circumflex above it - ô - occurs only in Slovak.
The letter U with an acute accent above it - ú
This occurs occasionally in Czech, at the beginning of a noun, but is much more common in Slovak.
There are many other ways to distinguish, but these are the best ways for a person who hasn't studied either language.