Make sure you have a good reason for the check. It will only be accepted to counter vandalism or disruption to Wikimedia wikis. Valid reasons include needing a block of the underlying IP or IP range, disruptive sockpuppetry, vote-stacking, and similar disruption where the technical evidence from running a check would prevent or reduce further disruption.
Be specific in your reasons. Ambiguous or insufficient reasons will cause delays. Explain the disruption and why you believe the accounts are related, ideally using diff links or other evidence.
Make sure there are no local checkusers or policies.
Reason(s): Repeated removal of maintenance templates at w:ro:Florin Alexandru Stan, likely motivated by promotional reasons. Used to circumvent the 3RR and escalated warnings. Andrei Stroe (talk) 18:40, 15 January 2017 (UTC)
@Andrei Stroe:It looks like a duck to me. Both accounts were created today in order to make the same exact edit, and that is obvious enough even without the CU. :) RadiX∞ 18:50, 15 January 2017 (UTC)