How do I check for C++20 support? What is the value of __cplusplus for C++20?

It's too early for that.

Until the standard replaces it, use:

#if __cplusplus > 201703L
  // C++20 code
#endif

since the predefined macro of C++20 is going to be larger than the one of C++17.

As @SombreroChicken's answer mentions, [cpp.predefined] (1.1) specifies (emphasis mine):

__cplusplus

The integer literal 201703L. [Note: It is intended that future versions of this International Standard will replace the value of this macro with a greater value.]

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The macros used, as of Nov 2018, are:

• GCC 9.0.0: 201709L for C++2a. Live demo
• Clang 8.0.0: 201707L. Live demo
• VC++ 15.9.3: 201704L (as @Acorn's answer mentions).

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PS: If you are interested in specific features, then [cpp.predefined] (1.8) defines corresponding macros, which you could use. Notice though, that they might change in the future.

The value for C++20 is 202002L, as you can see at [cpp.predefined]p1.1:

_­_­cplusplus

The integer literal 202002L. [ Note: It is intended that future versions of this International Standard will replace the value of this macro with a greater value. — end note ]

Therefore, for compilers that already implement the new standard, you can check by:

#if __cplusplus >= 202002L
    // C++20 (and later) code
#endif

As of 2020-09-04, this is the compiler support:

• Clang >= 10
• GCC: No (as of trunk).
• MSVC: No (as of version 19.27, note that it will require /Zc:__cplusplus).
• ICC: No (as of version 19.0.1).

For those that do not implement it yet, you can instead use:

#if __cplusplus > 201703L
    // C++20 (and later) code
#endif

Since all compilers define it already higher than C++17's 201703L in their respective "C++ latest" mode.

There's no known __cplusplus version yet because C++20 is still in development. There are only drafts for C++20.

The latest draft N4788 still contains:

__cplusplus

The integer literal 201703L. [Note: It is intended that future versions of this International Standard will replace the value of this macro with a greater value. —end note]

As for checking it, I would use @gsamaras answer.