Thanks to a tad of research and some very helpful answers, I now know that Jedi find Force-sensitive younglings through medical reports taken inside the Republic's boundaries. Jedis can also sense a Force-sensitive nearby (say within half a mile maybe?- or can they not?...I don't know- that was a guess), and can seek out out an individual they specifucally know through the Force. What I'd like to know is if a Jedi could, if they wanted to, meditate to discover new Force-sensitives over a large and/or specific area?
- The Force can be used to search for and locate individuals, even from across great distances
- The Force can be used to sense the presence of nearby individuals, especially if they're Force users that you've encountered before
- The Force can presumably be used to conceal your presence from others
As Yoda put it in The Empire Strikes Back:
YODA: Difficult to see. Always in motion, is the future.
What a person can or can't sense through the Force ultimately depends on
the needs of the plot the will of the Force.
In The Clone Wars TV show we see that the Force can be used to sense the future and thus locate individuals through the Force.
From the episode Children of the Force:
Yoda, Obi-Wan Kenobi, Mace Windu and Skywalker meditate, trying to detect any children through the Force, as many are being overshadowed by the dark side. Skywalker then heads to Naboo to secure a Gungan child and Kenobi to Rodia.
YODA: A jungle world. Dome cities I see. Rodia it is.
KENOBI: A house in Kay Tap Square. I see it too.
WINDU: There's an ocean planet, home to Nautolans, Glee Anselm?
YODA: Glee Anselm I see not.
SKYWALKER: I sense A place I've been to before. Waterfalls. Naboo.
YODA: Jan-Gwa. A village, southern sector. Sense it strongly, I do.
SKYWALKER: The Gungan child was screaming.
YODA: The future you see, young Skywalker. To Naboo you must go. Cad Bane you will find. With you, take your Padawan.
WINDU: You are certain there is no child on Glee Anselm?
YODA: Into the shadow of the dark side taken the child was. Shrouded is his fate from us. Great the loss is. But more children in danger there are.
KENOBI: I shall leave for Rodia immediately.
YODA: Find more younglings we will.
These Jedi used the Force to locate the Force-sensitive children. While it is doubtful that such an attempt to locate someone through the Force would be successful every time, it seems that it was the will of the Force in this situation that they be successful.
The Force can also be used to sense the presence of individuals nearby. The nearby presence of a Force user you've met previously can be especially perceptible through the Force.
In A New Hope, Vader senses his old master's presence:
VADER: I sense something; a presence I've not felt since...
In Star Wars: Rebels "Siege of Lothal", Ezra Bridger recognizes Vader's presence from having encountered him earlier. Vader, in turn, senses the nearby presence of his former padawan Ahsoka:
EZRA: There's something familiar... I feel cold. I think I know who it is! Back on Lothal I felt something. Kanan did too. The fear, the anger, the hate... It's the Sith Lord we faced!
VADER: The apprentice lives!
In Return of the Jedi, Vader and Luke sense each other's presence nearby.
LUKE: Vader's on that ship [...] I'm endangering the mission, I shouldn't have come.
Later on the Death Star:
VADER: A small rebel force has penetrated the shield and landed on Endor [...] My son is with them. [...] I have felt him.
Later on Endor:
LUKE: Vader is here, now, on this moon. [...] I felt his presence. He's come for me, he can feel when I'm near. That's why I have to go. As long as I stay I'm endangering the group and our mission here. I have to face him.
Individuals can use the Force to conceal their presence
Sensing others through the Force is not an exact science. Palpatine was able to mingle with the Jedi undetected during the Clone Wars. The question Why couldn't the Jedi detect Senator Palpatine as having powers? indicates that Force users can presumably conceal their presence using a Force Stealth.
From Revenge of the Sith:
YODA: The Dark Side clouds everything. Impossible to see, the future is.
Several Jedi such as Kenobi, Yoda, Jarrus, and Tano all managed to disappear undetected by Vader or Sidious after Order 66, perhaps indicating they each used Force Stealth or some other method of concealing themselves.
Theoretically, it's possible.
In Rebels, Ahsoka and Kanan joined forces and "remembered their training" to probe Darth Vader in the midst of a dogfight - probably kilometers apart - to assess his strength in the Force.
Searching for untrained Force sensitives is the same principle, but with more time and effort needed to succeed. Although a new Force sensitive may not be hiding his/her presence in the Force, he/she is not actively using it either. This is actually quite similar to the principles behind a more passive form of the Sith's Force Clouding technique in Legends:
The first and most common method was channeled through the mind. This was done by the individual being him or herself and allowing those around him to take them for granted. The practitioner would submerge themselves in the unified pattern the Jedi were attuned to; visible in the Force but not as a Sith.
In other words, you may be Force sensitive but as long as you don't use it and behave like you don't know you're Force sensitive, you can't be detected by anyone not actively searching.
So a new Force sensitive isn't completely invisible like the Sith, but his/her "blip" in the Force is still easy to overlook unless you're actively searching for one. Therefore, it is likely that when a Jedi specialised in perception and premonitions through the Force goes into meditation, detection of Force sensitives in an untargeted area sweep should be feasible, with the range likely related to the length of time in meditation and the Jedi's skill.
Remember, the Jedi Order started as a spiritual cult before midichlorians (the basis for detection through medical checkups) are discovered!