Tenant calls on neighboring tenant about violations

[Steven J.]

If I'm getting calls from only 1 tenants about drug activity, or people living in a unit not on a lease, what is the best way to proceed? Do I out right go and ask fellow neighbors in the same apartment building if they've noticed the same or is that 'fishing' for evidence against the suspected tenant in violation of the lease?

Its my assumption that I have to actually find plausible evidence, not just 1 tenants complaints', to move forward on notices of violation.