The IRS will know to look for the recording of the sale because they receive a copy of a 1099-S. Not all property sales produce a 1099-S form, but when it does the IRS has that record.
Prior AMT depreciation is the amount of depreciation for special depreciation allowance or an accelerated depreciation. If you did not do that, then the AMT depreciation will be the same as the accumulated depreciation number. The accumulated depreciation is the amount of depreciation that should have been taken even if you did not take it. That amount is 'recaptured' and you pay taxes on it.
You can only amend taxes for one year of missed depreciation. If more than that see this FAQ for your options and this post from AmyC on how to accomplish.
You might consider filing an extension while you get it all sorted out, but you'll have to do your best estimates of what you might owe in taxes and pay by 4/15 even if extending to file.
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