お客さまを迎える前には、トイレの鍵がかかるかどうかもしっかりチェックして。

、。

Before greeting guests, make sure to check whether the toilet lock works properly too.

Literal

Honorable-guest [obj-を] welcome before [to-には], toilet [gen-の] lock [subj-が] lock-[whether-かどうか] [also-も] firmly check-[request-して].

鍵がかかるかどうか 'whether the lock locks' embeds an indirect question using かどうか. しっかり 'firmly / thoroughly' is a common mimetic-derived adverb. して at the end is the casual imperative te-form — a light command or instruction. お客さまを迎える is the full keigo hospitality framing.