Hello, how can you tell the difference between the conditional and the imperfect tense in the first conjugation? The stem seems to sound the same for both.
Yep. They've both merged sound-wise into the conditional mood. It is worth noting that there is some difference in parts of Connemara still.
As galaxyrocker said, in most dialects they are the same, even those where they're not the difference is extremely subtle:
As you no doubt know, the -f- is pronounced as a "h" in some dialects, and in some dialects this causes the previous consonant to be devoiced, so you actually have:
This can lead to the rarest sounds in the language, devoiced slender r: