Required field problem

[cornish]

I have a data base which contains employees (about 650). Initially I has the
employee number field set to autonumber (primary key), however occasionally
it would skip a number (when there were two users in a network situation).
Having payroll numbers out of sequence (eg a missing number) created
problems with other software we use. To overcome this I have removed the
autonumber field and just set it to number (with a required entry control).
How can I make it that the user is reminded as they tab out of the field
that a number is required rather than wait until the last field on the form
is entered before getting an error message?

Any help appreciated (in relation to either problem mentioned above)

Thanks
Doug