Before you vote for a BP you should ensure they have funds and are registered to your rewards proxy. Furthermore, you should constantly monitor that their funds do not run out or they unregistered from your rewards proxy. If they do you should un-vote them immediately and notify them.
Check BP Funds
You can check if a BP has funds and are registered by visiting the Proxy dashboard page and clicking on your rewards proxy. From this table, you can see a list of the registered BPs and how much funds they have. For simplicity, you can also directly vote from this page by logging into Scatter with your Proxy account.
Additionally, each day you will receive an onchain notification that you can view via any block explorer that will notify you of the BPs that have sent funds that day and what the total amount was that was sent out to your voters. For example this was taken from bloks.io for Colin Rewards:
It is good practice to match up the BPs that have sent funds with the BPs that your proxy votes for. If there is not a perfect match it means a BP has either unregistered or run out of funds and needs to top up. You should unvote them and alert them.
Soon to come will be automated voting options for Proxy owners that would like to automate this process - stay tuned!