Suppose I know an account number, say user313948.
Is there any way to check all questions/answers posted by that user?
Yes, with this Data Explorer Query (enter the Id without "user" part). As I did not find any posts by user313948 specifically, I used another deleted user for illustration purpose.
From this comment is seems that you were interested also in some additional information:
That was user no in MSE. I am just checking if there are any posts in MO. user number will be same for both right?
Although it is digression from the original question (which is already perfectly answered), I might at least add something to this - which might clarify some confusion.
I am not aware of a simple way to find other deleted profiles in case I know one of them. (See also this post on Meta Stack Exchange: Will my deleted accounts be linked?) In this specific case I was able to find Mathematics profile and MathOverflow profile of this user in Google Cache. (In both cases, the cached version is from 15 March 2018. If I understand correctly how Google Cache works, the content you can see there now is likely to be removed at some point in the future - to be more precise replaced with the new version which essentially contains only the information that the account does not exists.)
So we see that userid on MathOverflow was 100241. The SEDE query returns no posts. (The same query returns 151 posts for the Mathematics user.) In the Google Cache of the MathOverflow profile I see three posts displayed among "Top Posts": Prerequisites for reading characteristic classes, How does one introduce characteristic classes and References on principal G bundle and connections. Since all of them are now associated with your account, I assume that the old account (with the id 100241) was at some point merged with your current account. (Maybe the moderators can confirm whether this is indeed the case. Or maybe you can tell whether this is the case - if you requested the merging yourself.)