@Neda Navidnia First off, the onus of due diligence should not fall on you, the agent, unless you have such a strong relationship with the seller that you don't believe they will sue you if the asset doesn't perform as you've suggested it will. From one agent to another, I can't stress this point enough.
That said, as an agent, you should help facilitate the deal and provide your input on the due diligence items provided, assuming you have the appropriate expertise. Financial reports provided by the seller are a good starting point but contrary to what @Danger Brown suggested, I would not take them at face value. They're a starting point from which to refine the financial analysis of the asset. Owners can easily pad the numbers to make an asset appear to perform stronger than it actually dues (ie. free rent or other incentives to lease up a property which won't appear on the pro forma)
Yes the tax return will be some indication of the actual performance of the asset but it's still not something you can completely rely on. Major items that WILL impact the new owner's financial results vs. the old owners include a new tax basis based on the sale price of the property (normally higher than the previous transacted price), deferred maintenance, changes in management, lease up incentives, etc.
Steps you can take to corroborate the numbers include comp sales, comp rents from property managers in the same area with similar units rented, inspections by licensed inspectors to determine useful life of major cap ex items, checking with county assessor/tax collector's office to determine new tax rate based on sales price, etc. This advice should really be provided to your investor but it's a good education for you the agent none the less.
Hope that helps and best of luck.