Verified answer

Like otaku fest. said, it should be 信じたろ

ろ could be a shortened form of だろう (used in Tokyo region, I guess...)

which describes "I guess/know/suspect ....."

信じただろう？ You believed that, right?　--> 信じたろ？

驚いただろう？ --> 驚いたろ？ I know you got surprised.

泣いただろう？ --> 泣いたろ？ I can see you cried..

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@cheapest: That's what I meant, actually. ろ is a shortened form of MALE colloquial ending だろう that describes "I guess/know/suspect .....", "....., right?"

Unisex/Female: ～でしょう？

Yeah, cheapest. As you see the above examples, I meant GRAMMATICALLY "ろ？" came from "だろう？" .(both with/without "?") "I guess" in colloquial, doesn't need to be translated "～だと思う" literally. If you still doubt, check Japanese translations here: http://eow.alc.co.jp/search?q=I+Guess

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I think you are talking about something like this

それは昨日言った

それは昨日言ったろ

Well, this ろ does not mean anything. It's just making this sentence more assertive in a masculine way. You can use some others like

それは昨日言ったね

それは昨日言ったよ

それは昨日言ったわ

etc

Addition:

@rhubarb

Actually, ろ is used to make the sentence more assertive. Not make it like "I guess,,,". I think だろう？ and ろ are different. But if you use ろ with ？, or with raising tone at the end, then it will be the same with だろう？.

信じだろ doesn't sound right to me.

It should be 　信じろ。　or 　信じたろ。

or　　信次だろ。(That must be Shinji.)

well... I'm not sure about 'ro' but this is what I found about verbs ending in 'ō'

Volitional (Presumptive):

examples:

走る hashiru (run) 走ろう hashirō

読む yomu (read) 読もう yomō

泳ぐ oyogu (swim) 泳ごう oyogō

Usage:

In general, the volitional form expresses intention, such as in these cases:

- In volitional ("let's" or "I shall") statements: 勉強しよう benkyō shiyō: "Let's study" or "I shall study".

- To ask volitional ("shall we") questions: 行こうか ikō ka: "Shall (we) go?"

- To express what one is thinking of doing, via 思う omou: 買おうと思う kaō to omou: "(I) am thinking of buying (it)".

- In the form しようとする shiyō to suru: be about to or be trying to. 犬が吠えようとしている Inu ga hoeyō to shite iru: "The dog is about to bark."

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