Wow, these are very sharp questions, allow me to put some armor on. you ahve 3 questions:
Q1: Can the listing agent tell you the highest bid on the property?
A: It is not ethical to do so, however the homeowner can allow the listing agent to disclose it to drive up the price. i.e. the latest offer is 550K, we need you to offer 560 or we are taking the 550K - tha tkind of situation.
Q2: is there any law....?
A: See above, the agent legally only owes the seller his/her allegiance. For example, the agent CAN NOT say, "the owner will take 550K" UNLESS the owner said it was okay for the agent to disclose that info. If an agent discloses situations like deaths, divorce, etc to a buyer - without the owner's permission, the agent can be held liable.
Q3: How do i find out the highest bid...?
A: Just in case you are going somewhere with this question, i want to answer that the highest offer may not be the best offer. I ahve seen many deals go for a lower offer b/c the buyer was mortgagable for instance. Or that buyer 1 can close in 30 days and buyer 2 takes 90 days... it depends on what the seller needs. Both situations are very different. the seller could be buying a new construction and his clsoing day may be 90 days away - so buyer 2 is better - even if the offer is 5-10K less, he may save that from rental, moving costs. Or, he may have to close/move for work and the 30 day deal is most cost-effective in the big picture - i.e. if the seller closes in 30 days the kids can start school in time.
The highest offer is NOT always the best offer.
Now, that being said - if you want to find the highest offer, you can ask - but the agent does not have to disclose. Once it is closed, signed, sealed and delivered then they can disclose the price as it is public record.
Hope that helps.
The phrase that agents use to desribe allegiance is: "fiduciary responsibility" link below: