I am in the process of losing weight and I am using a meal prep service to help me with portion control. I choose the ingredients, order online and they deliver via fedex. One of the options is to order grass-fed beef and, naturally, there is a premium on this meat (and the meal prep service is not cheap to begin with!).   So, when the meal is in a cooked state, how can I verify that they are sending me grass-fed beef? Are there any markers/taste I should be looking for? The service has a good reputation, but I just want to be able to verify that I'm getting what I paid for.  
To answer the question you asked  There are differences in flavor. There is no way to explain to you how grass fed beef should taste, since almost nobody has the skill to "read" a taste from a verbal description (proffessional tasters have a system, and the training to use it, which comes close to it). But if you taste both side-by-side, and concentrate on recognizing the taste differences, you should be able to say, "beef A is not the same as beef B".   To address the situation as a whole  I am surprised that your goal is "want to be able to verify that I'm getting what I paid for". People who pay more for eating grass fed beef do so because they prefer the taste of grass fed beef, and that preference is so strong that they pay a premium for it. So, if what you are interested in is taste, the most sensible thing to do is to taste both side-by-side, and to decide if you like the one called "grass fed" so much more that you want to eat that, regardless of the higher price. If you notice no difference - either because they are selling the same beef under two different names, or because your palate is not discerning enough - or if you notice it but don't find the grass fed preferable - you can stay with the standard quality beef.   There are of course cases where it makes sense to disregard taste, but they are not culinary motivated, and are mostly explainable through behavioral economics. For example, if you are motivating your dieting by an ambition to turn into a "sophisticated eater", and associate grass fed beef with the stereotype of a sophisticated eater, you may choose to consume grass fed beef as a kind of private Veblen good, regardless of taste. If this is your motivation, the authenticity of the beef would be indeed more important than your personal taste preference. There are also other alternative explanations in which your taste wouldn't matter. In those cases, there is nothing you can do on your own - you would have to either find a way to trace the beef provenance (maybe the seller can produce a paper trail that the beef was sourced from farms known to only go the grass fed route?) or pay an expert to appraise the quality of the beef you are buying.  