You would definitely know if you were on the 10-year route - as your letter would tell you so. If it doesn't, you are automatically on the 5-year route.
- For a start, you can't enter the 10-year route unless you are already living in the UK on a settlement visa (or other type of visa). So a fiance or spousal visa applied for outside the UK cannot be issued under the 10-year route.
- The 10-year route is an 'outside the rules' visa that is only granted in cases where the applicant is already in the UK and cannot meet the financial requirement when they renew their visa... and they must have a human rights case why they cannot move their family back to their home country (usually a UK citizen child living in the UK).
- So, if it was the case that you applied for a spousal visa from the US and could not meet the financial requirement, your visa would be straight-out refused with no option to qualify for the 10-year route.