hi! there have been a few questions on this; check these out for previous responses
Had we not found the Rosetta Stone how screwed would we be trying to translate Ancient Egyptian?
If we had never discovered the Rosetta stone, would we still have deciphered Egyptian hieroglyphs?
Without the Rosetta Stone could modern cryptographers decipher hieroglyphics?
Exactly how influential was the Rosetta stone in deciphering Egyptian hieroglyphs?
Well, if you happened to guess that hieroglyphs had a phonetic component you could try something like brute force for combinations of characters looking for common strings that could match place names or common words in etymologically related languages (primarily coptic) and seeing if those values produced anything sensible anywhere else (given, again, knowledge of etymologically related languages) - it'd be pretty rough, though. For one thing, everyone thought hieroglyphs were picture writing for the long time (I honestly think half of the people out there think that even today), you'd really have to make a totally random leap of logic to go down the phonetic route with no Rosetta Stone to guide you. It'd help a lot if you could identify the context of any one document - Linear B, for instance, was decoded because someone guessed that certain words only present in documents from Crete referred to Cretan place names, most of which had survived the fall of Mycenean Greece, which gave him the phonetic values from the script, and he was able to decode the rest because of it turned out to be a parent of Ancient Greek.
This would only get you understanding the phonetic parts of the script, though. There are other symbols that match morphemes and whole words. I'm honestly not sure how you'd ever solve that, without the clear knowledge of context of at least some part of the language a translation provides.
As the other links mentioned, there were other translations available, I'm talking if we had absolutely no translations at all.