not often any warning signs, some general advise for thos that are worried though
firstly go round and check all your tanks, make sure they are lined up on the stand and not over hanging, even a couple of mm overhang can cause problems. if the tanks are not floating base then get some polysytrene underneath them, make sure they are on as level a surface as is reasonably possible.
secondly make sure you are prepared so you can do the best for your fish if this does happen.
keep a spare tank or a large bucket big enough to hold a tank full of fish (obviously it'd only be short term but assume that it'd be sod's law and would crack when you've either got no money or it's xmas or something like that so no shops are open, assume the fish need to live in it for a couple of weeks to choose a relativley decent size. I like garden tidy tubs for this, if you've a few tanks or really big tanks you can stack these so they don't take a huge amount of room to store but you could give all your fish some space still.
keep a spare small filter and heater, nothing too fancy, just a half decent internal filter or something like that. only gonna be an expense of £30 or so which isn't the end of the world.
make sure you're nets are big enough to catch your fish. might sound stupid but plenty of people buy big fish as juvi's, they grow up then a year later they need to catch them and realise the 3" net is not going to cut the mustard! keep a big net on hand, also useful if you've small fish for catching them fast. we've a giant net, when i needed to get fish fast i just swept through the tank with the big net and got several fish at once.
and particularly if you've got cat's/children, something to cover the spare tank/tub with, again doesn't have to be fancy, just a sheet of ply will do. but something to stop the fish getting out or anything else getting in if need be.
just about being prepared for every eventuality isnt it.