The easiest way to confirm someone’s address on the inside in a state facility is to search on Google for “[state name they are in] inmate locator search,” using just their DOC number. VineLink.com is one website that might be helpful. For people in federal custody, you’ll want to use the BOP Inmate Locator.
For people in immigration detention, to find out if or in what facility an immigrant is detained, you can use the ICE’s Online Detainee Locator. The easiest way to use it is to input their A-number (the 9-digit file number used by ICE and the immigration court to keep track of someone’s case) and country of origin into the fields. You can also use the person’s full name, date of birth, and country of origin if you don’t have their A-number. When you do so, you will have to answer a simple math equation for verification. You can do this at any time, but it’s especially helpful if you don’t receive a response from your pen-pal after two months or so.
Another way to check on an ICE detainee (or formerly detained person) if they aren’t showing up in ICE’s ODLis the EOIR Automated Case Information Hotline. Unlike the ODL, the ACI only requires that you enter the person’s A-number. Using it, you can find out the person’s next court date (if they have one). If their court date suddenly changes to a date over a year from now, then it’s likely they’ve moved from the detained to the non-detained docket and thus have been released. If it says “No case found for this alien” or if it shows that they were ordered deported several months or years ago, then unfortunately it is likely that your pen-pal will be removed from the US if they haven’t been removed already, because ICE can use a prior removal order to deport someone without giving them the opportunity to see a judge beforehand. The ACI works better when you enter the A-number with dashes (i.e. XXX-XXX-XXX) instead of without (i.e. XXXXXXXXX), while the ODL works better without the dashes. You may have to refresh the ACI page several times before it shows the details of someone’s case as it is glitchy.