Figure out your target demographic/niche then ask those people in a general kind of way. It can get complicated though if it's something people don't know they want yet though in which case you'll have to proceed with caution.
As a developer I would suggest it's always worth exploring regardless of whether or not people find it useful - use it as practise and understanding how to develop and deploy a mobile application. Or, if you already know all that try playing around with new ideas/frameworks/designs to expand your knowledge base and deepen your understanding.
I'm currently in the process of building a mobile application myself. So I understand the worry and potential fear of wasting ones time. I knew my niche because I'm in that niche and have conducted my market research. This concluded that my idea is worth investing the time into. The only problem is that it's taking longer than expected/desired to build haha. But I've set a realistic date of releasing before xmas this year.
Send me a message if you'd like to discuss further - I'm also in the hashnode discord server :).