Category talk:Pages to Review Language
Sometimes one can try to narrow down the choice of which language some text is in from observing which accented characters are used. For example, an a tilde character is used in Portuguese yet not in Spanish and an n tilde character is used in Spanish yet not in Portuguese, so looking for the a tilde character and the n tilde character is a way to distinguish between Portuguese and Spanish. Portuguese also has an o tilde, yet so does Estonian.
The following web page has links to lots of pdf files, each of which shows the characters needed for one of the languages of Europe: the links are about half-way down the page at section 1.2.2 Alphabetic index of languages.
The information in a particular pdf does not permit one to say that some text is written in a particular language, only that it might be in that language or that it is not in that language, yet those can be very useful conclusions to be able to deduce.