returns -1 if no data is available.

According to the reference document and regarding

The function returns "The first byte of incoming serial data available (or -1 if no data is available). "

I believe the portion within parenthesis may be incorrect. If receiving binary data (non-ascii) and the first incoming byte is 0xFF, will return -1 since 0xFF is -1 for a signed byte.

So, I have found that it's best not to check if the returned value is -1 to determine if data is available or not.

The code snippet as written works fine. But unless I am misunderstanding something, I would suggest removing "(or -1 if no data is available)" as data IS available, it just happens to be 0xFF. returns an int.
There is no problem telling whether you got -1 (no data) or 0xFF (valid data).