This is a good question. Unfortunately I am not sure that there is a good answer.
We sort of guessed, but guessed in ways that seemed very highly likely (and it worked out).
One safety was an in-state public university with an acceptance rate a bit over 50% where the stats for the student were well above average for all accepted students. Another safety was an out of state public university, but where a large proportion of the admitted students are out of state, again with stats that were well above average, and where the NPC predicted merit aid (this was UVM, so there just aren’t all that many in-state students because Vermont’s population is so small). This was not really a safety however because we needed the predicted merit aid for it to be affordable (fortunately the predicted aid was spot on in our case).
The other safeties for our daughters relied on the somewhat unusual situation that our daughters were born with dual US / Canadian citizenship, and we live fairly far north in the US and therefore not all that far away from Canada. In Canada admissions is very largely stats-based, and both daughters had stats that made Canadian universities mostly safeties.
Frequently the guidance counselors at your high school will be very familiar with admissions at in-state public universities, and possibly at some popular universities nearby. They can frequently help to identify safeties.
And a student can use whatever tools your high school uses to show acceptance rates among previous year’s graduates from the high school at various universities (I think that Naviance might be one example if I am remembering this correctly).
Occasionally here on CC we see students whose proposed list includes a long list of reaches and some likely schools, and no safeties (or at least nothing that we think would be safeties). We frequently recommend that students add one or two in-state public schools even if they do not intend to go there, just to make sure that they have a safety on the list. We also frequently see students underestimating the quality of their in-state public universities.
For graduate programs frequently there just isn’t a safety. In our family, out of the four of us, we all went to a graduate program (three completed, one graduate degree still in progress). I do not think that any of us had any safety at all among the graduate programs that we applied to. Our safety was just to keep working at our current job for another year. Perhaps we all just got lucky.
There are a lot of cases where a school that is a safety for both admissions and affordability is also the best choice for a student. In our family, out of the four of us, three of us got their bachelor’s degree at a school that could reasonably be considered to be a safety. All nonetheless did very well, got a great education, and went on to a very good graduate program. There are a lot of very good universities in the US, more elsewhere, and you really do not need to attend a fancy hard-for-admissions university to do very well in life. The safeties sometimes are also more likely to provide merit aid, which of course can help with affordability.