In all honesty, there is no human or human endeavour that is 100% reliable, but you can get some that are significantly reliable.
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As for a developer, one that has the following qualities is significantly reliable:
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1. Good track record. The easiest way to know that a developer is credible enough is to look at his track record. How many projects has he done? Does he have abandoned projects? How solid are those projects? What are the experiences of his clients? etc.
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2. Reviews. What are his past and existing clients saying about him. A satisfied client might stay silent, but an unsatisfied one will blow a negative trumpet as loud as he can. And the news will most likely be online (if you search).
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3. What is his business worth and what constitute his net worth? A business worth several millions or billions of dollars in unmovable assets is most likely not going anywhere. Such developer has more to lose than you, so he has a reputation to keep.
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4. His financial records: you need to know if his finances are great or he’s been struggling. An experienced and financially buoyant developer will most likely handle your project better than a financially struggling one.