This really depends on the area. For certain parts of Ada county, the market value of homes is worth more than the assessed value and in other parts of the county the actual market value is lower.
The tax assessed value has traditionally lagged market value. In Ada county for instances it took them two years to catch up with market values and by that time the market was already moving backwards and they were assessed above market value. Needless to say, I don't envy assessors, they can never win.
Bottom line, your best determination of value is going to be through an appraisal. That is what the lender is going to base their decision off of. If you're a cash buyer, it still may be worth having an appraisal done.
If you're just trying to get an idea of value, your agent can pull the most recent comparable home sales for the area and should be able to give you a pretty good idea of what market value is.