There are several ways to check the categories of competitors' products on AliExpress:

1. Search for competitors' products: Go to the homepage of AliExpress, enter the name of the competitor's product in the search box, and the detailed information of the product will be displayed in the search results. On the product details page, you can view the category information of the product.

2. Analyze industry data: Log in to the background of AliExpress, enter the "Industry Analysis" module, and select the corresponding market analysis report. Through the data in the report, you can understand in which categories the competitor's products are distributed.

3. Observe competitors' stores: Go to competitors' stores, browse their product lists, and analyze the category distribution of products. At the same time, you can pay attention to competitors' hot-selling products to understand in which categories they have advantages.

4. Use data analysis tools: With the help of some external data analysis tools, such as Niu Mowang Cross-border E-commerce Data Analysis and Product Selection Expert, you can obtain sales data of various categories on the AliExpress platform. By analyzing these data, you can understand in which categories competitors have strong competitiveness.

5. Analyze the overall data of the platform: Through the "Product Management" and "Data Report" functions of the AliExpress backend, you can view the category distribution and sales of the AliExpress platform as a whole. Combined with these data, you can find out in which categories your competitors may exist.

In summary, through the above methods, you can initially understand in which categories your competitors' products are distributed. On this basis, you can further analyze the strengths and weaknesses of your competitors and adjust your own product strategy to improve your own advantage in the competition.