How does one handle this?
I swear that all of the 0-level humans that my players have hired have STR penalties. One beloved henchman (now a fighter with a spear) actually has a Strength of THREE! (He eventually took to one-handing the spear with Weapon Finesse, though.)
So, when hiring henchmen, do you reveal their stats up front? Hint at them generally? Surprise the players after the character is hired?
I suppose this is an area that is very open to Judge interpretation and it probably doesn’t hurt either way, but I’m curious how others typically do this.
I never reveal(ed) henchmen stats. I gave/give general hints, such as looks thin, sickly skin color, clumsy, wiry build, plain looking…
I reveal henchmen stats in the same way as I reveal magic items stats, e.g. when dice are rolled against them in play, or tasks using them come up in play.
Until then I try to roleplay or narrate their effects.
I think that there should at least be some indication of henchmen abilities. Strength 3 is hard to pass off as strength 13+. For ease of play, players should at least know henchmen stats when used in combat. If stats are kept hidden, players might demand auditions and tests for henchmen to determine stats and while this might be interesting the first few times, it can get boring fast(how many games of “how much weight can you hold so I can determine your strength score” would you like to play?)
Auditions seem perfectly reasonable. For a fighter, bar-fighting / wrestling to test Str, darts to test Dex, and a night of heavy drinking to test Con sounds like a decent job interview, and that’s very routine stuff for most adventuring parties. Likewise, engage the wizard-hench in a game of wits, riddles, or chess to determine Int, and have any number of battles of piety with your cleric candidate (holy book-quote-offs, fasting, jumping through fires… pretty much anything can be construed as ‘more favored by the gods’). If these sort of tests were used as a basis for hiring, you could tell PCs the relative stats of their henches (stronger than you / as strong as you / weaker than you) with no drop into meta.
That said, we play with open hench statblocks, even before hiring. Because we’re bad, and assume that it’s fairly easy to determine ability scores by observing people 
Who says open stat blocks are bad? I’ve got enough stuff to keep track of without managing the PCs’ personal armies, too. Besides, I want to give my players the information they need to be able to make meaningful choices. Leave the fumbling around in the dark in the dungeon where it belongs.
Same here. I leave them completely on the table. There’s enough stacked against the PCs that you need to obfuscate the mechanics of their closest allies and confidants.
The only piece of information I don’t reveal to the PCs is their alignment. Everything else is on a spreadsheet character sheet that I share with the hirelings ‘boss’. The onus for managing the hireling is on the players, though their reactions to things is oftentimes done by me.
What I started doing was that 0-level henches didn’t have stats. They were just one line stat blocks which the players dealt with. Then, once they were invested in them, and they hit level 1, I rolled the attributes. I realize this might have made some disconnect but it never seemed to cause too many problems.
Anyway, I let players keep track of their own henches stats. I don’t have time to track 20 NPCs and the monsters too!