So that we are clear in terms of why I ask, how do you know, here is why. Beltran has been in the league for 20 years. He has played for like 9 different teams and lets all remember that when you suggest that he learned this from the Yankees, lets remember that he was with Houston long before he ever went to the Yankees. He could have, and probably did, learn this long before he ever went to the Yankees but the bottom line is we don't know. Betran in NY, was a disaster both times.
In his first stint, he hit .233 with 15 dingers and .276 with 19 homers. You would think that if he were stealing signs, he would have done a lot better when with the Yanks, at the time. The Yankees themselves were 20th in the league in hitting in 2014 and 2nd in the league in 2015 so I'm not sure that we can draw any conclusion on where Beltran learned how to steal signs the way the Astros did. He might have, I don't know but if he did, you would have thought that he, himself would have enjoyed a much better season in 2014 and that the Yankees would have been much better as well. In 2015, the Yankees were much better but Beltran himself was not much better. Still a down year and in a park that he should have lit up.
To me it's really hard to point to the Yankees and suggest that it was the Yankees who started this. Now, could they have been doing it? Yes, that's possible but there is no conclusive evidence to suggest this IMO. That is, unless somebody has provided proof that they were stealing signs as well.