Hmm, here is some advice I have heard given, by people who have had problems:
Ask about their background. How and why did they come to do this job? Is it because of a genuine interest in home education, or is this a truancy officer who has been drafted in unwillingly to plug a gap in staffing? I think you may be able to tell straightaway in a phone conversation whether they actually like and approve of home ed and know much about it. Ask what books they have read about home ed, whether they've been on any training courses, what they know specifically about aspects of home ed which might be relevant to you - autonomous education, ASD, etc.
Don't be put off or reassured by other parents' experiences of this particular person. Frequently, people have wildly different reactions to an LA EHE visitor, according to whether the person understands their children, approves of how they educate their children, and so on. He or she may be very supportive toward some families and downright bullying toward others. Few staff are universally disliked or universally welcomed. You have to decide for yourself.
Decide whether meeting the EHE visitor involves any risk to your child. If so, consider meeting without the kids the first time, so you can decide whether you trust them to meet your kids another time. (By "risk" I mean, for example, the possibility that the LA person may say something undermining: telling the kids their reading isn't up to scratch, or trying to persuade children to return to school against their wishes, or telling loners they must join group activities, or saying that the child will be sent back to school if the education is found wanting.)
If you think you might feel intimidated, invite another local HE parent to join you for moral support, and as a witness to what happens.
Mention that you are in contact with other HE parents, or have a printed copy of the government guidelines with you and let it be seen. This will indicate that you cannot be easily misled or pushed around.
Ask for a written report afterward. Sometimes people find that LA records of the meeting are dramatically different from the parent's understanding of what happened!
If you think it at all likely that the LA might ever take you to court, don't accept a home visit. HE barrister Ian Dowty observes that if you do so, the LA can select whatever "evidence" they like to use against you, while ignoring anything that doesn't support their case. They can misinterpret what they see or could even fabricate things. If you submit written information instead, you are in control of which evidence is presented. (Very few families are ever taken to court, so I wouldn't want to make anyone unduly paranoid. But if you have reason to believe you might be targeted by the LA, a visit seems risky.)