The comparables method
The benchmark is recent listings — not sold, not active for three years, but posted in the last 60 days for comparable units in the same neighbourhood. Aim for at least 5–10 comparables to neutralize outliers (one luxury unit on the street can throw off any average).
Then adjust up or down for: square footage, bedroom count, floor and elevator availability, year built, recent renovations, parking (indoor is worth considerably more than street), in-unit laundry, balcony or yard.