Free. Check where and when you served to see which VA presumptive exposure rules may be worth looking at, with VA's own wording and links to its current condition lists.
For certain exposures, VA presumes the link between your service and some conditions. If you served in a qualifying place and time and have a condition on VA's list for that exposure, you generally don't have to prove the connection with a medical opinion. You still need a current diagnosis and records showing the qualifying service.
Check every place you served. You'll see which VA exposure rules may be worth looking at, in VA's own words, with links to VA's current condition lists.
Dates are VA's qualifying windows. Check any that match your service, even briefly.
Based only on what you checked. This doesn't decide eligibility: VA looks at your service records and whether your condition is on its list.
Next: open VA's page for each rule, check its current list of conditions, and talk with an accredited VSO (free) about filing. If you're filing, the free Veteran Roadmap walks through the steps.