The first question to ask is how long do you expect before rescue. If it's six months or less, then don't bother to sample the local food, it isn't worth it.
If you're sure rescue will come within 7 months then still don't bother. Everybody goes a bit hungry for 7 months and then you get rescued.
Within a year? Maybe you can survive on half rations for a year, if you don't need to do a lot of strenuous work. It's worth the attempt.
If you don't expect rescue in time, then you have a problem. Did the life here evolve separately and it looks similar by coincidence? Then you probably have very little chance. Suppose for a moment that there is basicly only one way that things evolve, and all the biochemistry is basicly the same. Then there's still the matter of chirality. Many chemicals have a left-hand and a right-hand form. Very likely each different one could be either form, although maybe some combinations don't work for life. Call it two possibilities for sugars, two possibilities for amino acids, and two for nucleic acids, that gives you 8 different forms before we consider anything else.
If they are the wrong form you can't use them. Some of them could seriously get in your way. At the very best you might find a way to take the food and convert half of each necessary ingredient to the other kind, and then you can use half of it and hope the other half doesn't cause too much trouble. It probably won't be easy to get that far. And that's the very best case.
You do far better if somebody in the past 20 million years spread your kind of life to various places including this planet. Then the fundamental biochemistry will be the same, and what's different is just what's evolved different since it got here.
Then your problem is not much harder than people going to a new continent, like the English in Australia. Except that there won't be any aboriginals to tell you what's edible.
It would help a lot if sombody brought along a bunch of lab rats. Or a truly sophisticated biochemistry lab. Failing that, get some plants and some volunteers.
Give each volunteer a plant sample. They taste it. If it doesn't taste too bad, they hold it in their mouths for a few minutes and notice anything that's there to notice -- uncontrollable salivation, a numb feeling, pain, etc. They spit it out and carefully rinse their mouths with water. Next day, if they didn't suffer, try chewing and swallowing a little tiny bit. Over about a week, you get a sense whether each volunteer's sample has actively poisoned them. Of course there could be slow poisons, and for those you just have to hope. Try to avoid the things that taste worst. Your senses have evolved to detect some terran poisons and they might be useful here too. And if you can't tell what will poison you slowly, you might as well avoid the potential poisons that taste the worst.
If everything you find appears to be poisonous, you might try the old way to eat it anyway. Chop it up and boil it in plenty of water. Throw away the water and boil it again with new water. After 4 or 5 rinses, water soluble poisons will be much less. The remaining solids may have a lot of food value. Or maybe not. If you don't have anything better then it's worth a try.