shakushinnen,
Check the bolts on the stand and the bolts that attach the board to the stand for rust. Check the board for cracks at the bolt holes and at the fulcrum. This is the point that the board flexes most.
If you have cracks at at either of these locations on the board then it is time to replace.
Most bolts will have some rust and stand bolts may not be visable if you have a fiberglass stand.
Rusted bolts will present you with a judgement call. Lube the bolts and remove to determine the amount of rust damage to the threads. Good luck.