I reckon your want to do this automatically. You can use e.g. the Stanford Parser (http://nlp.stanford.edu/software/lex-parser.shtml) which tags every word using the Penn Treebank tagset (for the verbs this includes information on the tense).

This is a first step, but of course it is not sufficient. The sentence might not be the best unit if your corpus is made of texts with reported speech or stories. Groups of sentences in past tense might refer to description of visions about the future, so adverbs and introductory clauses like 'I wish' are to be taken into account.

One can determine the tense of a given verb in a sentence. But multiple verbs referring to several tenses can occur in a single sentence. An interesting discussion here : http://contextors.com/The-Contextors-Tense-Conjugator

Patrice awesome tool, do you know the accuracy?

Not precisely...

I was happy to see that the Contextor website accurately pointed out that English does not mark verbs for the future and therefore does not have a future tense. Happier still to see that it accurately noted that aspect, involving progressive and perfect verb forms, is not a tense. I have long been puzzled as to why traditional grammarians and handbooks insist that English has 12 tenses.

@ James

"English does not mark verbs for the future and therefore does not have a future tense."

That's true. And we have the same in German. -

@ all

But I am astonished about the discussion. Mariana's question was: "Does a sentence refer to the past, the present or the future?" Past, present, future - that's TIME. But all contributors to the discussion speak of TENSE. Tense is not time! A sentence like "See you tomorrow" refers to the future. So does "Close the door!" The reference to the future is not bound to a certain tense. The same with the reference to the past and the presence: "John was born on June 13 in 2014 at 21:40 h" in a novel means the present time but uses the past perfect tense. In the web I found a headline: "Julius Caesar Crosses the Rubicon", written in present tense, but referring to the past time.

@ Patrice

"multiple verbs referring to several tenses" - that's not correct. Verbs may perhaps REFER to times, but they are BOUND to a tense.

Klaus correctly pointed out that we contributors did not construe the question in its literal sense, probably because we've had so many experiences with our students who ask this question but who are actually trying to understand tense. This is certainly the case in my linguistics classes. Some languages, such as Chinese, do not mark verbs for tense/time but instead use adverbials. Hence Klaus' "See you tomorrow." This approach is quite common in spoken English but is far less common in academic English. So my answer to Mariana's literal question is that all English sentences (and I suspect sentences in all languages) refer to time (past, present, or future) out of necessity, for we are caught up in time and cannot escape it physically or linguistically.

Something interesting concerning our question:

There are sentences, too, which have no reference to time. This is what Frege points to saying: „the thought, for example, which we expressed in the Pythagorean theorem is timelessly true“. So in such cases „is true“ is not referring to any time, it refers to the past, to the present, and to the future at the same time.