This something that a competant PCB Fabricator should already check for during their DRC checks within the CAM software they use for tooling the imported customer supplied data.
Having said that, the designer can help out by adjusting the soldermask where necessary on likely problem areas such as fine pitch QFPs.
Use the "By Layer" feature to adjust the size of the soldermask for the inner pins to remove the web between the pins and using a different sytle for these inner pins so they overlap each other; thereby ending up with a "block".
The alternative method is to simply draw a rectangle over the pads on the solder mask layer of course but the method I descibe ends up with single rectangular apertures that can be adjusted more easily by the fabricator.
I have uploaded an example footprint showing this.