I used to be a formulator of commercial vitamin supplements, so you might be suprised that I'd suggest that if you can eat normally and regularly and get at the very least 5 servings of fruits and vegetables a day, you might be fine skipping a multivitamin altogether. Or taking a specific nutritional supplement for a specific indication for a limited time, not a multivitamin every day.
The problem with folic acid supplementation in progression of tumors and cancers is not so much that it is somehow toxic but that it can mask vitamin B12 deficiency. If you've got some condition that limits stomach acid, or if you take antacids regularly, or if you have had colon reduction, then there is a special problem with excess folic acid.
The only way I'd know that you get an "early warning signal" of possible ill effects of excess folic acid would be unusual fluctuation up or down of your homocysteine levels compared to a baseline, which most doctors don't take. But I also think you'd have all your bases covered if you just got those fruits and vegetables, and that specific antioxidants (again, in moderation) like the curcumin from curry would be more helpful than a multivitamin.
And I used to go round and round with companies on the subject of iron fortification. Most men and women past menopause just don't need it, and nobody should take supplemental iron without a blood test showing they need it. On the other hand, your body won't absorb the iron in a fortified cereal very well unless you happen to eat it at the same time as you drink something like orange juice. The phytates in the cereal block the absorption of the added iron (now doesn't that make a lot of sense).
I don't mean to be glib, and I certainly don't know everything. I hope this helps, and I'd be glad to put on my thinking cap if it doesn't. Best wishes.