How do I know for sure IF I ever find a block using solo mining I am actually being paid 50 BTC (or 25 BTC later this year)? It would be a real joke if you find a block and get nothing? Where is the warranty to be paid IF found? And will it be really be paid to the bitcoin client address, not some random address???
When you find a block, it includes the transaction paying 50 bitcoins to you. If someone changed the address, the block hash would change and no longer meet the difficulty requirement, creating an invalid block. The only way the block you mined can ever be used by anyone is if it makes it into the public blockchain, in which case your 50 BTC payment makes it in too.
You might think, "well, maybe someone else will mine a block that replaces mine". But why would they? If they mine a block that comes after yours, they get the same 50 BTC themselves. And because their block would include yours in the chain, it would provide a longer chain and thus one more likely to get confirmations. (Longest chain wins, more or less.)
So as soon as you find a block, every miner greedily tries to find a block *after* it because that makes it more likely their blocks will "stick". A block must stick to pay the reward. A block cannot be tampered with -- a new block would have to be found to replace it.
Of course, there's always a risk somebody mines a block at very close to the same time you do and someone else mines a block after the other person's block rather than yours. Then you lose.