You won't be able too if it is a large, fairly competent, first rate agency doing the monitoring. Their resources are just too vast, and they can employ a greater depth of surveillance techniques.
If you've noticed anything, it's far more likely you have been monitored by a private investigator or police.
Telltale signs, well, heh, there are quite a few but they are hard to determine unless you have an exceptional memory. For example PIs who want to blend in will use white cars because (in the US) white cars are the most popular car color. You won't recognize a white car in an urban or suburban area as the more number of cars present, the higher (potential) number of white cars present and the less likely one white car will stick out to you.
However, in rural areas, especially where roads can get rough, white colored vehicles are harder to keep clean and thus much, much, rarer. A person who was tailing you in a white car would automatically stand way out to you in a rural setting. You might not consciously get why it does, but you would notice it and remember it. And if you had a really good (eidetic) memory (and/or were an avid amateur photographer) you might be able to remember it indefinitely.
There are many more examples, but to make it short and to the point, surveillance techniques tend to be tailored to locations and culture. If you can, you need to go back and forth between different locations to see if you can spot what stands out.
A person in a business suit in an immaculate car sticks out on a back country gas station. A college kid in Converses, skinny jeans, and hoodie sticks out in a red neck filled farm supply store. Likewise a cowboy boot and massive beltbuckle and cowboy hat wearing guy would stick out in a Hot Topic. A leather and jeans wearing biker type would stick out in an Ikea.
Mix the places you frequent up a bit. Try to go for places that are polar opposites to each other and go there one right after the other. They will get what you are doing after a bit, but initially they won't.