How can you find out if someone is scamming us for money?

How can you find out if someone is scamming us for money?
If someone offers to do a job for you or sell you something you must never ever give them cash or deposit up front. Guaranteed you wont get the item. If you want soeone to do a job for you usually only if the job is large may the builder for example may ask for 20% upfront so it helps towards materials. Even then they should of given you a written quote and price and show you the receipts for whats spent. You will also have labour charge seperate. If you take on someone with a day rate make sure that they do a full day, eg not turning up at 10 and leaving at 4 or four. You must get atleast 7 hours out of them. hope this is of help