Checking A Number;

I have a simple question. Can you check if a number has been passed through a form? For example I making a website for a property letting company and they upload the property themselves. But if you you put in text into the rent field, it throws up an error when adding to database obviously.I do validate the in put before adding it to the database so was thinking about another IF statement checking that a number was entered before they get as far as adding to the database and throwing up errors