I’m unsure of how to verify that though. 

It's also almost impossible to verify if they are in fact telling the truth.

Closing will take place 30 days after the units are verified vacant by you.

You didn't verify the rental income before you closed on the property?

Four of the duplexes (8 units) are rented out at an average of $450 per month (FYI, all info I'm giving is per agent and has not been verified as of yet) and the other two duplexes (4 units) are completely gutted and need all work done on interior.

Just put in a contingency for an acceptable appraisal.If your concern is whether it ia good deal put a contingency in for a due dillegence period and ask for all appropriate documents to verify income and expenses.

Did they verify income and call previous landlords for reference?

. $5000Total investment: $30,480Mortgage payment: $530/month (Conventional 30yr at 4.6%)Annual rental income: $21,000Annual Expenses:  Property tax  $1200 (verified through property appraiser website)Insurance $1,200 (best guess)Prop man.$1,680 (8% of rent)Mortgage $6,360Calculations:NOI = Income (minus vacancy)- Expenses (Prop tax, insur, and Prop mgt)         = 21,000- 4,080          = $16,920Cash Flow = NOI - Debt service (mortgage)                  = 16,920 - 6,360                  = $10,560ROI = Cash flow/ Investment basis       = $10,560/ $30,480       = 34.6%Cap rate = NOI/ Purchase price                = 16,920 / 129,900                = 13.0% Cash on cash = ???

Also depreciate half of the building if your units are identical. 

@Matt Shields since you buy auctioned properties, how do you verify if they have liens on them?