It really depends on the quality of the silicone at this point. You can feel the silicone and if it is hard, stiff, or crumbly, or missing in any spot, it would need to be resealed. If you are looking at a tank online or aren’t able to feel the silicone yourself, I would recommend buying it assuming it will need to be resealed
One of the tanks has a chip in corner eventually looks like the glass has cracked around chip. I checked the silicon and its still quite soft. The other one is fluval tank and they usually have a lot of silicon on and its rounded but that one feels quite soft still. They've been through a lot like hot and snowy conditions.