But did they tell you about where your Spanish ancestor came from? I'm asking only because this phrase or idea is common among nearly every single filipino. I hear it with a lot of Filipinos today, just as it was in my own family too, and it's the same thing. If it's not quoting/citing their parents, they say that their grandparent said it. In our family too, it was, "Grandma said that she had some Spanish." I knew that couldn't be the case and I proved it to be true.
Not unless you came from an area in the Philippines where the few prominent spanish families were from like the Tagles, Arrastia, Reinares, then that's understandable. Even in the town where my grandmother was from, while going through the recs, I saw only a couple of foreigners, one in particular was a Mallorcan captain, and this was in the 1860s. So we're talking 45+ years after the trading stopped, and therefore it would be on rare occasions that some foreigners would make it to the distant colony of Spain via New Spain, which was governing the Philippines at that time up until 1821 of course.
But find out from your parents when, and who exactly. Was it their father, grandfather, great-grandfather, or something that they've heard about from other generations?
What you need to do is start off with what you know. Your grandparents, all of their names, their birthdates, death dates (if they're no longer alive) and when they were married and most importantly where all this took place.
Once you have all that info., you can begin to work your way backwards. A baptism/birth reveals the parents names, sometimes all 4 grandparents' names and their profession and/or origin. A marriage record may reveal their age, and parents' name sometimes and a death record may reveal their age, sometimes parents & grandparents as well, depending on the area and the church and the person who wrote the doc.
So start with death, if that's all you have, say of a great-grandparent and work your way backwards. Death date may reveal the age, you can look for a baptism rec. from there based on their age, and/or marriage record using an estimated date. Keep working your way back until you come across the foreign ancestor.
Also, what Spanish surname did your ancestors have? For example, not until I traced my grandmother's great-great-great-grandparents who lived in the mid-1700s did a friend noticed the name of one of her ancestors - BIONGSON, which he said it's of Chinese origin. In the recs. it identified him as a Filipino (indio) but he may have been just pure chinese, or half, I don't know. But his name suggested that he was of Chinese origin. But that's only 1 great-great-great-grandparent for my grandmother out of 24 that I found for her. I am missing 8 more of her great-great-great-grandparents for a total of 32 3rd great-grandparents.