I've stored some strings in web storage (session and/or local), and am wondering if it is possible to check for such stored strings on page load or init on the server-side (asp.net c# in my case)... So, for example, I will know not to re-fetch data from the db and use what is already resident in the browser from the last page load.
No, that's not possible. sessionStorage lives on the client. If you want to access it on the server you will have to write javascript that reads the value, puts it in a hidden field so that it is sent to the server. Or javascript that will read the value from the storage and redirect to the server passing it as query string parameter. There's absolutely no way for the server accessing directly this storage. That's one of the drawbacks of sessionStorage vs cookies.
来源：https://stackoverflow.com/questions/10488790/access-web-storage-from-server-side-possible