To be honest, you must be the first person I've heard say "I don't eat enough".
On a serious exercise regime ( even the pro team cyclists ), checking bodyweight on the scales is a daily ritual. Same time of day after morning dump, same clothing ( pref naked ).
If weight is down, eat more. If weight is up, eat less.
Of course the team's nutritionalist will have all the team member's personal BMR details from months of lab testing, and set out a menu which meets the days requirement.
If you get a chance to see a cycling team eat breakfast, it looks like a free-for-all bun fight, but its all prepared and counted.
Buy a set of good scales. Check daily and look at your musculature in a full length mirror. Pinch the skinfold at your lowest rib. As a rough guide, 1mm = 1% fat. That's a real 'rough' guide.
Pro cyclists have a skinfold there of about 10mm. Lesser cyclists can be 12 - 14 mm.
As for riding a bike on the public road, somewhere between 40 and 50 kCals/mile is a reasonable usage.
Bon apetite