Look to see what others say about them. I was looking at one dropshipper which had what I thought was some good products. But checking them out it turned out the products were crap and their customer service was appalling. Once they got the customers money they did not give a damn. And if you're dropshipping their product it is you who has to deal with them and will get it in the neck from your customers. And did I mention they were charging £125 per year for you to dropship their goods.
Focus on reviewing products that fall within your niche. Then, leveraging the rapport you have created with your audience and your stance as an expert, tell your readers why they would benefit from purchasing the product you are promoting. It is especially effective to compare this product to others in the same category. Most importantly, make sure you are generating detailed, articulate content to improve conversions.