Can’t figure out whom to admit and whom to discharge.
I like the old adage "If they want to be admitted, they probably don't need it. If they don't want admission, they probably need it."
On a more serious note, keep it basic. When it comes down to it, there's only a few reasons to admit 95% of patients.
1) They're a direct risk to themselves d/t a (treatable) mental illness
2) They're a direct risk to others d/t a (treatable) mental illness
3) They're an indirect risk to themselves because they lack the ability to keep themselves alive d/t a (treatable) mental illness
The first two are pretty self-explanatory. Are they altered enough to be a threat and you can do something about it? Admit them. Are the a threat simply because they're antisocial? Don't admit them (but make sure you're documenting why they can't be treated). The tricky parts are knowing the local laws or facility policies that can effect this. Whisper the word suicide at my VA and you're getting admitted (oftentimes even when you're not suicidal). Substance detox will muddy the waters (though it shouldn't), so ask your attendings about this.
The third one is when things can get tricky. How do you gauge whether they can adequately care for themselves and if it is d/t treatable mental illness? Get collateral if possible. Hasn't bathed in 3 weeks and is only eating/drinking olive oil (actual patient)? Admit them. Only bathing once a week, but still able to feed themselves and not has housing, probably not going to admit unless one of the first two is true. Are they so disorganized you can't even get an appropriate answer out of them and can't get collateral? Admit them.
It's also important to get good at assessing capacity for those patients who would benefit from admission but refuse. If the olive oil guy can hold a completely appropriate conversation, explain why he is doing what he is doing, and can tell you the risks of doing this, it's going to be a lot harder to involuntarily admit them than the guy who can't even string a coherent sentence together.
The other 5% of patients are either administrative/social admits or the very rare "wtf is going on?" patient when they're fine with admission. These include patients going to a substance or other specialized program in 3-4 days who you want to keep safe until then/detox, patients who have MH problems and don't feel safe at home (sometimes d/t their own mood, sometimes d/t someone else threatening them) until you can get them somewhere safe, or "the rocks" aka patients with significant MH waiting placement. In terms of the last one, these patients often tie your hands because you just can't safely d/c them but the resources they need aren't readily available, I see these more at the VA than anywhere else.
There's always caveats and exceptions to the above, but if you stick to the 3 rules above, you'll do fine with most patients. These can also vary depending on what other resources your area has available for patients (having good IOP/PHP programs around is helpful for keeping chronic self-harm patients out of the inpatient unit).