There's nothing wrong with talking to the current tenants to get their views about the house and the landlord/agent. Don't be put off by what they say- you can always ask the landlord or agent to get any problems they had sorted before you sign your contract.
Check the house is safe and has the basic features including;
Put any repairs or alterations into the contract and make sure they are added before the contract is signed.
Confirm the total charges payable to the landlord/agent, and also the weekly rent.
It is a good idea to ask about rent over the summer holidays, sometimes rent is halved over the summer as most people do not move in until shortly before the new term begins.