Numbers Only Input Mask, If Checking For Number

I have a control called GoToId which afterupdate moves the user to a specified record where ID=GotoID. I want the control to only take numbers as input. I used input mask: 999999;;" " but with it the user can still press a letter on a keyboard and even though the letter doesn't get typed in, I think an empty string gets passed as value.
If I can't do it with masks, what's the simplest if statement in vba that will check if the value is a number and positive? (IF the value is Null, "", or anything else I want the if expression to be false)
I know how to do it with several if statements checking if is null, or if "", but there must be a clean way of asking it in a single expression.