A very common question, that is easily answered via a self-assessment using the following 4 criteria / questions. 

Whether the following criteria are adhered to via a tracking or non-tracking / mindful eating method doesn't matter. Tracking might make assessment easier, which may be a needed to eliminate variables if tight control is required and/or if mindful eating dieting methods are not yielding progress. 

1. Calories. 
Is roughly the correct amount of Calories being consumed on average over time? For the goal of fat loss, this will be a deficit relative to energy expenditure, also terms an energy deficit, Calorie deficit or nagative energy balance. 

2. Sufficient protein
Is a sufficient amount of protein being consumed access a daily basis? For the tracking crowd anywhere between 1.4-2g/kg per day will suffice. 

3. Sufficient plant fibre
Current recommendations stand at 15-18g per 1000 Calories consumed [1]. Of that 80% should come from plant matter. Current recommendations for fruit and vegetable intake start at around 300g, & 400g respectively [2]. 

4. Mostly wholefood food selection. 
Most of your diet should come from minimally processed whole foods with the general rule of thumb being 80%+. 

So before you message every Insta diet guru (most of which don't know much about much) just self asses 'the diet' in question against the above. There you will find your answer.  

Bonus factors. 

1. Post diet transition to maintenance eating. 
Sustainability of a diet is surprisingly not a factor here as doing something slightly unsistainable is perfectly fine provided someone has the ability to healthfully, and effectively transition into the post diet maintenance free from rebounds, development of disordered relationship with food etc. For this to be obtained often basic diet know-how, autonomy and education is required. Which begs the argument of another important factor of a successfully diet being; education & autonomy. 

2. Ratios & amounts of carbohydrates & fats. 
Are sufficient carbohydrate amounts being consumed to meet needs? For most 'general-pop' folks who have low levels of activity, and low/ if any training volume and intensity carbohydrate needs are low so how you fill the rest of your Calories after protein has been covered doesn't really matter, so use preference. 
For high-level athletes or those with high training demands carbohydrate needs may be hugely elevated compared to the typical so a carbohydrate bias ratio is almost certainly the best idea with fat intakes only meeting minimum requirements of about 1g/kg of body weight, or 20% of total intake. Typically speaking there are more benefits to learning towards a carbohydrate bias in most situations. Your "body type" is not a factor to consider here.

3. Meal frequency, meal timing, and nutrient timing. 
Again for most people the number of meals you eat, when you eat them and when you time certain macronutrients (Protein, carbohydrates, and fats) relative to the day or training doesn't matter so dedicate your stress elsewhere. 
For high-level sports, or physique athletes and/or goals, or those who are very lean looking to get leaner timing of meals but more specifically protein feedings matters. Ideally, protein should be evenly spread across waking hours in even dosages. Carbs should be emphasised around training to promote best training performance for sports progression and/or optimal muscle growth / retentive stimulus. Timing of fats doesn't matter a whole lot, but I see value in going on the lower end of fats in the pre training feeding window. 

[1] Dietary guidelines for American 2015-2020

[2] Australian dietary guidelines