Start with what the water tells you
Water always finds its own level, which makes the tank itself the most honest gauge of the stand beneath it. Sight along the front glass: if the gap between the water surface and the rim differs from one end to the other, the stand has shifted, sunk or twisted. A small difference that has been stable since setup is one thing; a difference that is growing means something underneath is still moving. Look next at the line where tank meets stand: a gap opening at one corner, or the tank resting on three corners and floating slightly on the fourth, concentrates the whole weight onto the glass seams least able to bear it. Silicone seams under that kind of twist are what eventually let go, usually without further warning.
Signs in the stand itself
Cabinet stands made of board fail from water long before they fail from weight. Run a hand along the base panels and around the bottom edges inside the doors: board that feels soft, swollen, bubbled or crumbly has been wet repeatedly and has lost most of its strength, even if the outside still looks tidy. Doors that used to close flush and now overlap or drop on their hinges show the frame racking out of square. On metal stands, look for rust blooming at welds and feet, and paint blistering along the lower rails. Any stand that rocks when you press a corner, or creaks when the floor is walked past, is telling you its joints are loose. Finally, check the floor beneath: a dipping floorboard or a rug that is always damp points to the slow leak that has been feeding the damage.
Why you cannot fix it loaded
A full tank weighs far more than the furniture reasoning most people apply. Water, glass, substrate and rock together mean even a modest community tank presses down with the weight of several adults, concentrated through four small feet or one cabinet base. Shimming one corner of a loaded stand drives the twist into the glass rather than out of the stand, and jacking or dragging risks cracking a base panel that cannot flex. Repairs to swollen board are cosmetic; the strength does not come back. So the sequence is always the same: the fish come out to a temporary container with tank water and a running filter, the tank is drained and lifted empty, and only then is the stand repaired properly or replaced. It is a disruptive day, but it is a planned one, which a burst seam in the night is not.
Making the move safely
Line up help before you start: an empty glass tank still needs two people and a clear path, and the fish need their holding container aerated and covered in a quiet spot. Photograph your pipework and cable runs before disconnecting anything, and keep the filter media submerged in tank water throughout so the bacteria survive the move. Once the tank sits on its new or repaired stand, refill partway and check the waterline again before filling completely; this is the moment to catch a floor that slopes. If the whole job is beyond what you can lift or house, this is exactly the kind of heavy, awkward work local aquarium services exist for. You can find and message services in your area through the Pet Services Directory in KinsLe, and having the tank's details already on its profile makes explaining the job — size, livestock, access — a great deal simpler.