As Panagiotis Kanavos wrote as a comment to the question, you can check the dependencies section of the package details page on nuget.org to see which target framework(s) a package supports. However, it unfortunately only shows the dependencies as defined in the nuspec file, which theoretically might not match the libs in the package, but what's more common is when a package only supports a single framework, or doesn't have any dependencies, that the nuspec doesn't contain a dependencies section and therefore nuget.org doesn't tell us which frameworks the package has libraries for.
What you can do is take the url to a package detail page on nuget.org and change the n to a f to point to fuget.org. For example EPPlus 4.1.1's nuget.org URL is this:
https://www.nuget.org/packages/EPPlus/4.1.1. Notice how nuget.org doesn't list anything under dependencies. Change the n to an f to get this url:
https://www.fuget.org/packages/EPPlus/4.1.1. When you open that page, you see next to frameworks it says net35 and net40, so you can see that the package has binaries for .NET Framework 3.5 and .NET Framework 4.0.
Looking at the latest version of EPPlus, we see it has binaries for net35, net40 and netstandard2.0. While net35 and net40 are runtimes, netstandard2.0 is not, but if we look at the docs page for netstandard, we can see that netstandard2.0 is implemented by .NET Core 2.0, .NET Framework 4.6.1, Mono 5.4, and others.
