Thank you for reaching out to Univ. of IL Extension Forestry and thank you for the extremely dangerous work you do out there in the woods! I have a great deal of admiration for the logging community! You've asked an outstanding question and I'll do my best to offer several suggestions.
First and foremost, let me applaud you for being extremely responsible and professional by reaching out to the adjacent landowners -- job well done! It is truly unfortunate, however, that the adjacent landowner has rejected your responsible professionalism and has taken an extremely passive-reactionary position to your hospitable invitation to verify the property boundaries.
Here are Several Options that Immediately come to Mind:
1). Hire a professional surveyor to verify and establish corner posts and property lines. Flagging or blazing property lines every several hundred feet costs extra money due to extra time, but it will be a heck of a lot cheaper than a lawsuit and associated attorney fees and court costs.
IMPORTANT FACT: A landowner is allowed to deduct the full cost of a property survey on their IRS income tax return as part of their timber sale expenses. I’ll leave it up to you to decide the fairest way to split the cost of the property survey with the landowner.
2). Consult an attorney and send a notarized, certified letter to the adjacent land owner; this will indicate that you’ve taken the initiative to avoid any and all trespassing issues that may or may not arise due to future logging activities.
Hope this helped and feel free to contact me directly at (217) 244-Zero Five Three Four if you have additional questions or concerns. Stay safe out there!