You can find out if a journal is PubMed-indexed through the website of PubMed by searching something called “NLM catalog”. This is a link and above is a screen shot of the page. I noticed that you need to place your search terms between quotation marks (” “) in order for the desired journal to appear. Not using quotation marks will result in many unprecise results. I do not know why. For example, I want to search for the following journal: “Archives of Medical Research“.
94 results and the journal does not appear within the first 20 results (link).
Three results and the journal’s name appears the first (link)
Finally, after the journal’s name appears, click on it to move to its page. Look for the “Current Indexing Status“. Next to it, it will be written if the journal is currently indexed or not indexed for MEDLINE. Example of an indexed Journal: Archives of Medical Research (link):