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): Since the user bg:User:Mpb_eu has been recently voted for ban, but is expected to regularly return to break it, I anticipate that there will be a number of requests related to this user in near future (see also the previous). :) The request now is to check coincidence with bg:User:Полк. Дянко Марков, who exhibits identical behaviour and thematic scope of contributions. For bg:user:Христо Зарев Игнатов the community's suspicion is lesser, since the communication pattern is different, but again the thematic scope is identical. Also, I guess that checkusers do not perform checks for username/IP coincidences, but there are grounded assumptions for IPs starting with "212.5.158.", "212.5.131.", "220.127.116.11", "18.104.22.168". Any information that can help us handle these cases will be welcome. →Spiritia (BG WP sysop) 11:31, 19 May 2015 (UTC)
The IP ranges should actually be read as 22.214.171.124/18 and 126.96.36.199/24, which are used for the GPRS/3G clients of two Bulgarian MNOs, and thus are perhaps not very helpful (although I've noticed a tendency for the users who try to avoid bans to go for the mobile networks for some reason: probably because in that way they are guaranteed either a public proxy address or a dynamic one). 212.5.131.* is trickier, as it doesn't have a separate record, but since it's part of a larger pool, 188.8.131.52/19, belonging to one of those two operators, and the two neighbouring networks, .130.0/24 and .132.0/24, are assigned to different subjects, I assume it's used again for the GPRS/3G clients as 184.108.40.206/24. As for 220.127.116.11, it also seems to be a dynamic address, in the 18.104.22.168/20 network, which is operated by a (terrestrial) cable television provider. — Luchesar • T/C 12:35, 19 May 2015 (UTC)