Is there any way to read the belt contents of the miniloader? I have a requestor chest feeding my rocket silo via two miniloaders. The chest items get requested based on circuit signals from the destination planet and rocket silos, etc. But the problem is I can't measure what's actually on the belt of the miniloader, so it's a bit inexact. Any solution?

At this time there is no solution. Miniloaders are a combination of belt and inserters, and it is not easy to read both the belt contents and all the inserter contents.

Even if you could read exact Miniloader contents on a particular tick you would still have to compensate for combinator processing delays. The usual practice is to work around the problem of counting items in transit.