In my line of work, I do a lot of referring out to other professionals. I've done a lot of networking over the last decade and have developed a pretty broad network. The struggle sometimes is narrowing down which professionals to refer to. I created a little set of considerations I call the "5 C's of Referability" to help me when I'm choosing who to refer to:
Character
I like to say I don't really care how good you are at your job if you don't have integrity. For example, I know some real estate agents who are really "good" at their job in terms of getting clients to work with them and getting their offers accepted, but are totally slimy to work with. They'll lie, cheat, whatever they have to do to get their clients a win, of course, while earning a nice commission. When I'm considering who to refer to, someone's character is far more important to me than their ability to close deals.
Competence
To be clear, I do actually care about whether you're good at your job. You don't need to know everything, but ideally you have experience relevant to what my referral needs help with. It's ok to say "I don't know but I'll get back to you". It's not ok to think you know something, move forward with that assumption, and get something wrong for my referral. You've gotta know what you don't know, and either pass on the lead, or know who to get help from when there's something you don't know.
Communication
I like working with people who are good communicators, and I mean this in two ways- First, do you have the ability to explain what you're doing for my referral in a way that they can understand? And second, are you responsive when someone reaches out, regardless of whether they're a potential, current, or past client?
Capacity
Do you actually have the capacity in your current workload to take on my referral? I get being busy, I really do. But are you self-aware enough to recognize when you don't have the space, or the support, to take on another client right now? I much prefer a message letting me know you can't take on clients this month but to please keep you in mind for future referrals, than to refer someone to you only for them to get ghosted, or to fall through the cracks after being onboarded.
Collaboration
Do you work well with others? A lot of referrals I send out are related to projects I'm also working on with the client, such as planning to buy a home, dealing with the death of a loved one, etc. I love working with professionals who are willing to collaborate on those cases and communicate directly with me (with the client's permission of course). It can be a lot of work for clients to be the intermediary between professionals when the conversations are complex.
There you have it! Character, competence, communication, capacity, and collaboration- the key ingredients to a great referral partner. :)