When I mine via p2pool using the monero gui wallet, how do I know when I actually mine something? Will it appear in the Transactions tab?
Yes. P2pool will send you a transaction for every share you find. It will be small amounts of Monero but there it will be in your transactions.
You can also see the shares you have found on this site (this is for the mini pool)
https://mini.p2pool.observer/