How to use ending evaluation in UCI interaction with Stockfish Playing around with the Stockfish 8 UCI from the terminal, I've managed to make it compute moves, and set the position and load Syzygy tables.

But how I can know if an endgame, one that corresponds to the kind of endgame I've loaded, is winnable or drawable?

I've looked at the Stockfish UCI protocol, but I have not found the relevant information.
If the root position you set up on the board is covered by the syzygy tablebases you loaded, you can just directly query the tablebases using the Stockfish-specific (non-UCI) command d, see the example below.

Since syzygy tablebases are depth-to-zero (DTZ) and not depth-to-mate (DTM) (see https://www.chessprogramming.org/Endgame_Tablebases#Metrics for the definitions), what you get is the result with perfect play for the side to move as well as the number of half moves until the 50 move rule counter will be reset due to a capture or pawn move.

input:

setoption name SyzygyPath value {path to your tablebases}
position fen 8/8/8/5k2/8/1R6/R7/4K3 w - - 0 1
d


output:

info string Found 35 tablebases

 +---+---+---+---+---+---+---+---+
 |   |   |   |   |   |   |   |   |
 +---+---+---+---+---+---+---+---+
 |   |   |   |   |   |   |   |   |
 +---+---+---+---+---+---+---+---+
 |   |   |   |   |   |   |   |   |
 +---+---+---+---+---+---+---+---+
 |   |   |   |   |   | k |   |   |
 +---+---+---+---+---+---+---+---+
 |   |   |   |   |   |   |   |   |
 +---+---+---+---+---+---+---+---+
 |   | R |   |   |   |   |   |   |
 +---+---+---+---+---+---+---+---+
 | R |   |   |   |   |   |   |   |
 +---+---+---+---+---+---+---+---+
 |   |   |   |   | K |   |   |   |
 +---+---+---+---+---+---+---+---+

Fen: 8/8/8/5k2/8/1R6/R7/4K3 w - - 0 1
Key: 48F90DF481A6F1D6
Checkers: 
Tablebases WDL:  Win (Success)
Tablebases DTZ:    7 (Success)