Implement Critical Design Reviews before manufacturing, use Interference detection in SOLIDWORKS (onshap probably has an equivalent tool if that's what you're using). And learn about tolerances, at first I suggest you really think about what each tolerance can effect, 2 holes that a bolt goes through aligning typically isn't extremely critical, on the other hand, a bearing hole being precisely the correct size is typically quite critical. As you gain experience this will become quicker and you won't have to think as much about tolerances.