Annual return, this is most obvious, compare it to SP500 to make sure it exceeds.
Fund Manager, make sure the manager of successful period of the fund is the same manager now.
Expense ratio: net expense ratio is actual expense for last year (like actual sales price), gross expense ratio is expense would be charged (like a retail price).
It could get very complicated. That's why I suggest index fund or ETF. For fidelity it is IVV and ONEQ.