[TorontoCFE]

Well, to answer your questions (normally I attach the standard disclaimer that this advice is general and may not apply to individual situations so check with proper advisors but I don't like sounding like a lawyer, I prefer direct answers):

1. Canada Revenue Agency can find out about offshore / foreign accounts. However, they need to have a reason to look for one and ask for it from the other government. Relations with the US are much closer and there is some question as to whether there is routine data sharing there (for anti-crime purposes).
That said, they would have to have a reason to suspect something was going on. That would either come from a tip or an audit. In the case of an audit, the auditor is free to estimate your income (if there is reasonable proof it exists) and then the onus shifts to you to prove the estimate is unreasonable - auditors have quite broad power (in the past, an auditor estimated my client's tax due at $700,000 and we had it reduced to $25,000, but we still had to pay up and try to get the money back later).
I could go on and on about how what would make them suspicious or how one can defend oneself, but that is really best left to individual cases. Besides, I am in no way advocating tax evasion. The truth is that it is pretty unlikely the CRA would find out, but if they did then you could face not only tax liabilities but tax evasion charges.

2. One issue that people used to look at was how much time you spent on your hobby. In my opinion, it wouldn't be considered much of a factor any more. It really boils down to expectations. If you meet the factors I mentioned above, then it will be considered a business. Did you know that if you buy stocks, assuming they will quickly go up and intending to hold them for a little while and quickly making a few bucks, is actually considered a business and not a capital gain? - it all comes down to intentions.

If you approach poker as a source of income, I think it's taxable. By definition, a hobby is something you do without expectations of profit. Even if you spend one hour a day playing 200-400 poker, if you know you can beat the game and you pay your mortgage with your winnings, then I believe CRA will come looking for its share some day. The relative share of your income, whether it's 80k job, 10k poker or 10k job, 80k poker, doesn't matter.

3. I thought about this last night. If you make a large withdrawal / deposit, you may get your bank reporting you to FINTRAC, just like a CTR in the US, under the Anti Money Laundering / Terrorist Financing Act. Large deposits will stand out, especially if the bank doesn't know your income source.

On the other hand, if you make your money in large multitable tournaments, then you have a better arguement that it is a windfall and not a business. The problem is that it would be up to you to prove that. Tax-wise, you'd have a better position winning the WSOP and that's it for the year then winning $5 million playing 4 hours a day every day.
If I am making $50-200k/yr at poker, then I may look at setting up a CCPC (small private corporation) with its own bank account, depositing funds there and then paying yourself a salary. Then the poker earnings are taxed but at the lowest possible rate, you can control how much money gets to you and in what form, it is all legal and above board and prevents tax evasion charges - but thats just me.

If you really don't want to declare the income, that's up to you but it carries a risk - not much of one now, but the risk will grow in the next 2-5 years.