Methods for finding new Trace Flags in SQL Server
There's nothing you can do to find the list except ask or pick them up from posts/slide decks/etc. The list only exists in the code, in a header file where the valid trace flag numbers are mapped to names in a big enum in the C++ code, and then the names are used in the rest of the code.

As Aaron said, you can enable any trace flag number, and if it doesn't do anything, or you don't exercise the functionality that the trace flag is relevant to, you won't notice any difference in behavior.

DBCC TRACEON doesn't check anything - as there's no run-time list of which numbers are valid or not - it just enables that trace flag number in a bitmap of what flags are set for that connection/globally.

The problem with having a validity check is that would expose which trace flags are valid, allowing them to be discovered. This way the 'valid list' is effectively obfuscated, which is what the SQL team wants.

Regarding Kin's suggestion in a comment that SQL Server should have select * from sys.available_trace_flags - Yes and no. There are many trace flags that are highly detrimental to performance and are only necessary for debugging problems under guidance from Product Support, but SQL Server could list the 'safe' flags.