Bitcoin API - How can I identify new incoming payments?
If you want to identify incoming payments, the recommended way is creating a new receive address for every request. When the confirmations of a transaction to a given address get high enough, you can consider the payment confirmed.

Using the 'sender address' does not give you reliable information - it can only identify (one of the) addresses that were previously in control of the coins. There is no guarantee that these correspond to the sender of the payment (it may be a webwallet with shared coins, for example).

Not reusing addresses also has other benefits, like making bitcoins as a whole less easily traceable.