How would I know that it is ok to sign an agreement with a patent/invention company?

I would caution against it. Often these companies are more expensive than using a reputable company and perform very little in the way of a valuable service. Before signing, I would first check with the US Patent and Trademark Office to see if this is a company against which complaints have been filed and I would inquiry of the company as to what percentage of their clients have obtained an amount in royalties that is greater than the fees paid to the company. Also, you might wish to check with a local patent attorney to see what he/she would charge to perform the legal services that are part of the invention company's package of services.