Step 1: Turn it on. Did it let any of the smoke out? Good. You're on your way.

Step 2: Use it. Do all of the features you designed into it work? Good. You're still doing okay.

Step 3: Try operating it with every possible combination of inputs. Do any combinations make it do anything unexpected? Excellent. You're getting closer.

In all seriousness, there are a great many tests you could do. There's HAST (Highly Accelerated Stress Testing), burn-in testing, durability testing, but if you just want to make sure it works, just turn it on and try to crash it. Whatever the design, whatever the project, there are a finite number of inputs each of which has a finite number of possible settings. Write down each possible combination of input settings and try each of them. Then let it run for a day or so to see if it's going to overheat or lock up. If it still works properly the next day you're probably (stress probably) in pretty good shape. To say any more would require considerably more specificity in your question.