There is a suffix in Kyrgyz and I am not sure whether it denotes future tense or imperfective aspect. Are there any tests which can help me to distinguish between imperfective aspect and future tense?
The Max Plank Institute's Department of Linguistics has a few resources, including a questionnaire, for dealing with questions of tense and aspect. But as Dominik hinted, tense and aspect are a tricky thing, and a often very intertwined to the point that it gets quite difficult to pick them apart, and even linguists can disagree (for instance, many sources talk about a past and nonpast tense in Japanese, while many others talk about perfective and imperfective aspect).
You need to distinguish between tense, aspect and time reference. For instance, the English present continuous combines a present time reference with an imperfective aspect. However, it is often used to denote future time reference as well in some contexts. E.g. "We're leaving at 8 sharp tomorrow morning."
What you need to do is a functional analysis of the whole system of tense, aspect and mood. I tried to something like that for Czech, here: http://www.slideshare.net/bohemicus/presenting-aspect-as-tense
The whole system of labels for tenses and aspects is really a mixture of tradition and some general principles but their definitions are not reliable to predict actual uses across languages. For instance, in the above presentation I showed that you could easily reimagine a Czech system without aspect or the English system with only three tenses and three aspects. These things are just more or less arbitrary labels for ways in which languages code reference to time, perspective on action progress vs. completeness, repetition, mood, etc.
That said, I find the labels 'tense' and 'aspect' useful shortcuts when describing these complex phenomena - particularly in broadly related languages - but they're just that.
As the others have said, Tense Aspect and Mood are tricky. Their morphemes often combine meanings, and they imply even more. A language may not formally mark a category so it becomes convention to use another category to communicate the one that isn't marked. The Prominence of Tense, Aspect and Mood by Bhat is a good book on the topic. Check your library because it's way overpriced!
To precisely determine what the suffix marks will be hard, but there is one easy test: can it be used to communicate past continuous/imperfective? If so then I think it's straightforward to call it an imperfective rather than future. If not, then unfortunately the test won't help you know either way.