Question: I've been trying to wrap my head around the idea of stocks with future earnings/value "priced in". How does this work exactly, with a fund. Does some manager look at the possible earnings a fund might gain (over some period) and add that to the current valuation of the fund?

If that is so, what time period does a manager use?  

What research does a manager use to get the number?  

How can one tell if the "priced in" numbers are valid?

Answer: 
Anyone who wants to can use any method they want. Ultimately, the price of the stock will settle on the valuation that people tend to agree on. If you think the priced in numbers are too low, buy the stock as that would mean that its price will go up as the future earnings materialize. If you think it's too high, short the stock, as its price will go down as future earnings fail to materialize.

The current price represents the price at which just as much pressure pushes the price up as down. That means people agree it's reasonably approximating the expected future value.

Imagine if I needed money now and sold at auction whatever salary I make in 2019. How much will I make in 2019? I might be disabled. I might be a high earner. Who knows? But if I auction off those earnings, whatever price it sells for represents everyone's best estimate of that value. But each participant in the auction can estimate that value however they want.

If you want to know what something is worth, you see what you can sell it for.
"Priced in" just means that the speaker thinks the current price has already taken that factor into account. For example, the difference in price right before and right after a dividend is released often differ exactly by that dividend -- the fact that the dividend would function as a "rebate" on the purchase price was priced into the earlier quote, and its absence for another year was priced into the later quote.

The term can be applied to any expected or likely event, if you really think the price reflects that opportunity of risk. It just means that this factor, in the speaker's opinion, doesn't create an opportunity one can take advantage of.

(Tupos foxed. Darned auto-incorrect...)