HOW DO YOU KNOW? The short answer is, you don't.

"We're all fortunetellers is what we are," Pete Mackanin said Tuesday afternoon as he sat behind the desk in his windowless office at Citizens Bank Park.

The topic of conversation was one of the handful of wild cards who had emerged on his roster two months into the season. Specifically, the 26-year-old outfielder who two days earlier had connected on his fourth home run in 65 at-bats, a game-tying, three-run shot with two outs in the eighth inning of an eventual extra-innings win over the Nationals. For a Phillies team that had lost five straight and eight out of nine, Aaron Altherr's blast was perhaps the definitive moment of their young season. Muddling through the latest step in their arduous rebuilding process, in the midst of a hellacious stretch of schedule that saw them play 22 of 30 games against playoff teams of a year ago, a potential member of that far-off future stepped up to the plate and smacked the latest salvo in his bid.

And yet, there is irony. Because, years from now, nobody will remember Altherr's home run. Either he'll bury its memory beneath a long line of similar blasts, to the point that we'll someday forget that there was ever a question about how long he'd last. Or he won't, and we'll one day forget he ever existed at all.

Call it the Jayson Werth/John Mayberry Jr. conundrum. One is still smacking home runs for a World Series contender, the other is . . . doing something else. It's easy to forget that once upon a time Werth was every bit the unknown commodity that Altherr is now. Entering his 28-year-old season, he was a rotational outfielder with 232 career games, a .753 OPS and 25 home runs to his credit, a onetime prospect now regarded as little more than a flier. It's also easy to forget that, at one point in time, Mayberry offered every bit the potential: at 27, he returned from a stint in the minors to club 12 home runs in 163 at-bats over the last three months of the 2011 season.

Werth's career numbers at 26: 825 PAs, 25 HRs, .245 BA, .333 OBP, .420 SLG, .753 OPS.

Mayberry's at 27: 369 PAs, 21 HRs, .265 BA, .328 OBP, .518 SLG, .846 OPS.

Altherr's at 26 (entering Tuesday's game): 468 PAs, 13 HRs, .232 BA, .330 OBP, .407 SLG, .737 OPS.

They are not identical players. You can make a compelling case for a closer resemblance between Werth and Altherr than either of them and Mayberry. The first two were at one point legitimate prospects who ended up following similar paths through the minors, right down to the wrist injuries they suffered early in their prove-it seasons. Mayberry never showed the kinds of flashes that Altherr and Werth did while working their way up the ladder. Both look the part in the batter's box more than Mayberry ever did.

But at some point in time, at some similar juncture of their careers, all three players forced us to ask ourselves the same question:

How do we know whether this is for real?

It's the same question we asked about Domonic Brown during that ridiculous stretch that earned him a bid to an All-Star Game. It's the same question we asked about Maikel Franco two years ago, and, perhaps, it's the one we're still asking about him now. It's the one the Mets asked themselves before allowing Daniel Murphy to sign with the Nationals, and the one the Astros asked themselves before cutting J.D. Martinez loose to sign with the Tigers.

Is this who he is? Is this who he'll be? What kind of sense does this game make, anyway?

In no other sport does a guy spend seven years as a merely adequate player and then, at the age of 31, start to produce on a level with some of the game's all-time greats. But that's what Murphy has done over the last two years in hitting 200 points over his previous career .755 OPS.

"I always considered him a good hitter," Mackanin said, "but not what he's been doing. He's really figured something out . . . He's turned into a monster."

In no other sport does a guy do the equivalent of hitting 24 home runs in his first 899 at-bats and then, after earning his release, hit 23 in his next 441. That's what Martinez did, and it's what he's been doing for three years now. Yet for every Jose Bautista, there is a Dom Brown, and for every Dom Brown a Casey McGehee, and for every Casey McGehee a Justin Turner, and let's not even get started on Eric Thames.

Mackanin comes at it from an interesting perspective. Before landing his dream job, he spent much of his career at baseball's formative levels, including a couple of seasons as a scout with the Yankees. A couple of years ago, during the breakout portion of Odubel Herrera's rookie season, he spoke with confidence about the centerfielder's staying power, about what he saw in the swing mechanics, and the eye. Time seems to have proved him correct, but he knows enough to know that baseball time is a different beast.

"What you're really looking is ideally for a guy that gives you quality at-bats on a daily basis," Mackanin said. "When you find a guy like that, he's a special player and he's an everyday major league player. When I was here as a coach with the Phillies when everybody was hitting, Ibanez and Utley and down the line, they gave you quality at-bats."

He says that's what he's seen thus far from Altherr, particularly since the player made some adjustments to lower his hands and shorten his swing.

"You can see it," Mackanin said. "I think I can see it in the quality of his at-bats because of that. We've seen guys here with an uppercut swing that have really done well for a while, but then all of a sudden the opposition attacks your weakness, and if you can only hit one area, they stay away from that area, and all of a sudden a guy can't hit."

How do you know for sure?

You wait.

@ByDavidMurphy