First you would have symptoms and seek help for it. In the blood test performed at the ER, they'd look for a protein called 'd-dimer' (dee dime rrr) which is an indication that your system is attempting to dissolve a clot on its own. It's a natural waste product showing natural dissolving of a clot. If that is present in an elevated range, they'd know you have a clot, and they'd probably also know where the clot likely is, depending on what symptoms you're complaining about.
If you had a cerebral stroke, you would probably have symptoms and others would notice and encourage you to seek help. Sagging on one side of your face, inability to speak clearly or at all, can't lift one arm or the other above your head...that kind of thing. A clot in your heart might cause an infarction or just angina, depending on which vessel has become obstructed by the clot. Again, it's very likely to be objectionable enough that you'd know to seek help.