Using your county's parcel search like an assessor would
Every Illinois county publishes a searchable roll. Most owners never open theirs.
The same records the assessor works from are, in most Illinois counties, public and online.
What you can usually see
PIN, owner, site address, class code, land and improvement assessment, total assessment, and often lot size, building square footage, year built and a sales history. Some counties publish a full property record card with room counts and a sketch.
How to search for comparables
Most portals let you search by street or by neighbourhood as well as by address. Searching your own street returns your immediate block; searching a nearby street with similar housing gives you a wider pool. Note the PIN of anything promising — it is the identifier that never becomes ambiguous.
Read your own record card first
Before hunting comparables, read what the county says about you. Errors here are the easiest wins and you are the only person positioned to spot them.
When the portal shows little
Some counties publish assessments without characteristics, which makes building comparisons impossible and land comparisons still workable. Some publish nothing useful at all. In those, the Supervisor of Assessments holds the roll and Illinois FOIA reaches it — ask for the electronic file rather than a printout.