I'm assuming you have studied the matter of investing from a Biblical perspective? So then, if you are having trouble identifying what is an investment and what is speculative, you need to study the subject matter of investing itself in order to be able to compare the discernment issues to scripture.
Benjamin Graham has two famous quotes which are decent point of reference: "an investment operation is one which, upon thorough analysis, promised safety of principal and a satisfactory return. Operations not meeting these requirements are speculative". For many, this would be kind of the John 3:16 of value investing. Another decent quote from the same source would be "short term, the market is a voting machine, long term it is a weighing machine"
So then one foundational difference between an investment and speculation is the ability to analyze an asset or investment scenario, verses one which cannot be analyzed or is essentially random. For example, gambling is random. Speculation is a little different from gambling, but is still not investing.
Biblically, we are commanded to consider well our steps. Also, the underlying principle of Proverbs 11:14 would apply to investing as well - you need advisors, and you need to get sufficient depth and breadth of knowledge on an investment matter. So you need to avail yourself of expert opinion, one way or another. However, it is also biblical to not place total trust in anyone other than yourself, and to be very careful about who you would trust in matters involving money - which means you have to be sufficiently wise biblically to be able to search people out who you are dealing with, and also sufficiently educated on the subject matter. A subscription to the Wall Street Journal or the Financial Times is only a few hundred dollars per year, which is, no pun intended, an investment if you are planning to spend allocate some portion of your earnings to investing over the course of your lifetime. If you plan on saving and investing, which you should, it is foolish not to be as informed as you can be on the subject matter. Also, Bloomberg's website is free and is comfortable to both the WSJ and Financial Times.
In a nutshell, you cannot really boil down a definition of investing into categories such as "this asset class is investment quality, while this other one is speculative". Asset quality alone cannot be the yardstick by which you determine is a decision is a prudent investment or is speculative, because ultimately there is a price at which any asset becomes overvalued, and also a price at which most assets are undervalued. You want to try your best to get a good deal on whatever you are buying, and this would extend to financial assets as well. A lot of people make horrible investments by buying a high quality asset at too high a price. A lot of people make great investments by buying medium quality assets at undervalued prices. The ultimate success of an investment is obviously 100% price dependent.
So the biblical approach to investing would therefore be to learn how to approach and handle the matter as diligently as you would any other matter in life.