How do you know if you have a flea problem?

Hi I have 2 dogs and I was just wondering How do you know you have a flea problem? How do you know if your dogs have fleas?

I had my poodle at the groomer yesterday and she said she found signs of flea dirt on him. And asked if we had been using any frontline.

We don't use flea/tick preventative on him or our other dog, but especially him because he has such bad reactions to the stuff. And I don't feel comfortable putting those chemicals on my dogs month after month. I have never used flea/tick preventative (other than on him) on any dog I've had in the past and I have never had a flea/tick problem.

Now she didn't find any fleas or anything like that. We have 2 dogs one that goes to a groomer ever 12 weeks or so and one that never goes to a groomer, she is more of a self maintaining dog (terrier mix). The dogs are bathed weekly.


But we do live in an apartment and there are other dogs in the general vicinity. Is it possible my dog had been somewhere in an area where there had been fleas but didn't catch fleas? (from what I understand Its too cold now for fleas here)
How would I know if my dog has fleas?

What are the sure signs you have a flea problem?

(You would think we would be getting bit up if we had a problem)

Thanks

Should I go ahead and bathe both my dogs in a flea shampoo just as a precaution and what is the best.