Does the order of fields in SELECT query matter when using composite indexing?
No, you can specify the 'params' (the parts of the where clause) in any order and the query optimizer will handle it. The optimizer will do the filtering in the order that it estimates is most efficient, but note that this is more complex than just choosing which order to filter: filtering might be done before or after joining for example.

You can't exactly prove this, but you can demonstrate it is true for a particular query by experimenting and seeing if the plan changes. It may even be true that there are edge cases where the order does matter, but my advice would be to ignore the possibility and assume it never happens as otherwise you will expend a lot of effort trying different permutations. Much better to focus on the kind of tuning which you know can pay dividends (eg correct indexing).