Ideally you don't. Often if you ask the organizer they can find someone with a decent ear to help you get things balanced in the hall. While this is a bit risky (maybe the person they pick has strange judgement or wants to play with EQ settings) this has generally worked out better for me than not having it.

If you're in a larger band, one option that can work is taking turns having musicians go out into the hall and set levels. For example, in the Free Raisins (fiddle, mandolin, piano) when we ran our own sound we'd start the first set with just fiddle+piano, with the mandolin player (me!) in the hall checking fiddle-vs-piano and caller-vs-band, plus getting the caller EQ dialed in. When that was sounding good I'd get back on stage and the fiddle player (Audrey) would go out and set the mandolin level with the piano as reference.

In a duo, though, what do you do? One option is for the fiddle player to use a wireless electric violin, which lets them go out into the hall and hear exactly how it all sounds together. This seems ideal, but Ed Howe is the only person I've seen do that.

What I do instead is set up the board where I can reach it from where I'm sitting. Then I take the main speaker farthest from me and rotate it on its speaker pole until it's pointed at me. With all the other speakers (monitors and the other main) off I set levels, but I do this very differently from usual. Normally my approach is:

1. Set a rough input gain for each instrument by eye, using the input trims and the pre-fader-listen LEDs
2. Turn up instruments in the monitor as needed until everyone is happy.
3. Turn up instruments in the mains until it sounds right

That doesn't work well when running sound while playing, so instead I:

1. Set each instrument's mains levels to 50%
2. Set each instrument's monitor levels to 50%, with the monitor master all the way down
3. Set input gains for instruments by ear, based on how they sound in the single main speaker

When running our own sound I need the monitor levels to be an accurate guide to the mains levels: if something sounds wrong in the monitor, it probably sounds wrong in the mains too. I will adjust mains or monitors separately during the gig, but each time I do it I think hard about whether there's a good reason why they shouldn't be moving together (ex: a loud instrument on stage doesn't need to be in the monitors as much).

This is not perfect, and works better if the musician running sound plays entirely electric or electronic instruments (I have a hard time setting the level for my talkbox, and I think it was slightly too loud at Porchfest). Even if you don't have someone in the hall setting levels for you, if there's someone who can signal if you're way off that's helpful.