It's rarely the fires themselves that prevent access--oftentimes they are burning in less accessible areas that affect a specific system of trails far from the frequently used trailheads. For example, last year the PCT was closed near the Canadian border. The bigger problem is smoke making the hiking unpleasant and obscuring the views. The smoke might be from local fires as it was last year, or from fires in Canada, Montana, etc--the effect can be the same. If there are smoke issues, there are smoke forecasts and predictions you can look at to see which areas are relatively clear.