For the most part, the sign boards will tell you whether tax has already been included or is to be added to the price lists. Merchandise, inside of stores, rarely have tax included in the price.
In terms of sit down restaurants (e.g., Blue Bayou), those menus generally do not include tax. Likewise, at the counter service places (e.g., French Market), the prices also generally do not included tax; look at the bottom of the menu for "Sales Tax to be Added" type note (at least it's often at the bottom). For some of the kiosk type vendors (e.g., popcorn, Mickey bars), the prices are often included for simplicity. In general, it's probably easiest to assume tax has not yet been included.
At DLR, I've found that tax is generally already included in the prices of the items that come from the push carts (like the ones that are on the street before the parade), in vendors that are cash only (like balloons), and for some foods sold at cart/kiosks (e.g., popcorn, Mickey bars).
While not universal (so don't take this a hard and fast rule), if a listed price is a multiple of $0.25, there is a good chance the price already includes tax. For example, popcorn is either $4, $6, or $12; churros are $3.75; bottled soda $3.25. So things like those often include the tax already.
