A trail commission is an ongoing payment to the person who sold you an investment, taken each year from the investment for as long as you hold it.
In US mutual funds, trails usually come out of 12b-1 fees. FINRA lets a fund pay its brokers a service fee of up to 0.25% a year for ongoing service and account maintenance, and an asset-based sales charge of up to 0.75% a year on top.
The trail is built into the fund's expenses, so you pay it through a lower return rather than a bill, and it keeps being paid whether or not you hear from the person who sold you the fund.
Only someone registered to sell securities can receive trails, so a fee-only advisor doesn't. When you switch advisors, it's worth asking whether any fund you still hold pays a trail, and to whom.
How to check
The fund prospectus's fee table shows the 12b-1 fee that funds most trails. Search the advisor on FINRA BrokerCheck: a current broker registration means they can receive trails, and Item 5.E of the firm's Form ADV Part 2A says whether anyone there does.
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