Assuming you are seeing a speech therapist who has experience with stuttering, this should not be an issue at all. I'm a stuttering specialist and it is common for people to speak very or even perfectly fluently during our evaluation session. I always ask how today's fluency levels compare to your typical levels, and also what situations are your "triggers" where it manifests the most. By asking the right questions, a good therapist should be able to get a sufficient-to-start picture of your stutter, even if it isn't obvious in the session. If the therapist tells you, "Well I don't notice anything, so you're fine," go find a more experienced specialist.