They wouldn't be really able to see it unless they suddenly decide to audit your accounting.
If you're paid in crypto, you usually sell it for fiat with an exchange. There is no record of who deposited what to your account. All they can see is xxBTC have been deposited the last x months, xx BTC have been sold, and xxx$ have been transferred to the bank account (your).
Obviously, taxation with cryptocurrency isn't always clear, and I'd say in many cases a lot of people don't actually know that you have to declare your earnings. I know its confused me in the past. This is probably due to the varying rules, and regulations around it in different countries. However, I'm not sure claiming ignorance is the best solution.
The UK government at least now have it pretty clearly written on the government website what you need to declare. Though, even then there's some things which aren't spelled out for you. Hence, my recommendation for an accountant.
Honestly, I find it difficult to wrap my head around, since you have to get the amount that Bitcoin was worth at the time of the transaction, and then you have to work out how much its appreciated. I personally just like to hand that off to the accountant otherwise if I were to do it mistakes would be inevitable since math certainly isn't my strongest point. Yeah, this means relying on an accountant, but at least you know its being done correctly then. I receive payments in Bitcoin quite regularly, however I don't always take that out immediately. I guess if you were to take the amount out immediately it would make life a bit easier since there's little appreciate of value then.
I couldn't with a good conscious recommend anyone not to declare their taxes. As the saying goes, some things in life are guaranteed, death, and taxes. Although, I bet its quite common that someone didn't realise they needed to declare cryptocurrency when they haven't converted it to fiat, and then those people realise they need too, and will have to back date a couple of years, and pay fines if deemed appropriate.
Though, I suspect that the majority of users here on the forum aren't declaring their signature payments, and regular income via Bitcoin. Whether that's ignorance or actively looking to avoid tax. Like I said, I would absolutely always recommend everyone to declare everything even if its as you say; unlikely unless audited. I find honesty is the best policy.