Table Tennis Doubles Hand Signals
As far as I know, there are no universally agreed hand signals for table tennis doubles serves. You can just discuss with your partner before the game to agree on them.
Below are the ones that I find generally useful. The general rules are:
Showing pinky (last) finger indicates backspin
Showing thumb indicates topspin
Not showing pinky nor thumb indicates no spin
Showing index (second) finger indicates long
Not showing index finger indicates short
Some might indicate sidespin by pointing a finger left or right.
Some might indicate long by waving their hand forward but I find even when done under the table, the opponent can see the arm waving, giving away unnecessary info to the opponent. So I prefer to just use the index finger to indicate long or short.