Look at how much room you have around the tire as it passes through different regions of the bike; brakes, rear brake bridge, chain stays, and so on. Every 1mm of added tire width will reduce each clearance by around half a millimeter. Note that there's a lot of slop in tire sizes, they can vary by a millimeter or two from manufacturing tolerances and whatnot, so if you don't have much room, you might need to mount a tire before you know if it'll work. I personally also like to leave at least a couple millimeters of clearance everywhere to be on the safe side.

BUT, also.

25->28 isn't that huge of a difference, and there are other ways to squeeze extra plush out of tires.
Even if you can't go to 28mm, you could try out a high-performance racing-style tire in 25mm. And if you do go to 28mm, make sure to not get a harsh-riding tire, because that could easily more than cancel out any gains you get from the added size.

Also crucial is selecting tire pressure well. Don't err high.