Duden
The Duden page provides examples for each meaning of a word, from which you can deduce the case:
3. mit jemandem, etwas in Verbindung treten
Beispiele
wie, wo kann ich Sie erreichen?
ich habe die Firma, das Büro nicht erreicht
durch den Film erreicht man viele Menschen
"Sie", "die Firma / das Büro" and "viele Menschen" are neither genitive nor dative. Thus, it must be accusative case. (Nominative case is no option for the object of a verb.)
This is not perfect for a language-learner at beginner level.
Leo.org
Canoo.net (later canoonet.eu) had been an excellent resource for language information. Unfortunately it has been closed a few months ago. The new owner is not interested in maintaining the website and database. Some of its content moved to leo.org, a German-to-various-other-languages dictionary site associated with the university of Munich. Currently a huge wealth of information seems to have vanished - hopefully not for good!
In any case, the dictionary itself is a bit more useful than Duden. It doesn't explicitly name the case, but it shows the possible prepositions and associated cases. "jds." conveys genitive case (for example: "jds. Platz einnehmen" = "to take somebody's place"), "jdm." (= "jemandem") dative and "jdn." (= "jemanden") accusative.
- (jdn./etw.) erreichen (-> accusative)
- (jdm.) helfen (-> dative)
- (auf jdn./etw.) warten (-> accusative)
- sich (vor jdm./etw.) fürchten (-> dative)