There are several things to check actually.
Clustering factor is one of those things.. But the vast majority of indexes actually do not require rebuilding. There are lots of myths even about this topic!
There are conditions like;
Large free space (generally 50%+)
High Cluster factor (but if that's the case maybe need to rebuild the table not the indexes.. So it should be checked)
Size of index
etc.
The answer depends... There is no short answer is for this question.. Again! You should read and internalize ->
https://richardfoote.files.wordpress.com/2007/12/index-internals-rebuilding-the-truth.pdfAfter reading the document (I think that document is very informative and it is generally about index rebuilds and it is in the internals mode, giving you all the myths..) above, you can ask me questions then .. (if you have any questions of course)