Ferrofluids are colloidally stable suspensions of magnetic nanoparticles in a suitable carrier liquid. Because of their unique properties, such as magnetoviscous effect and a wide spectrum of possible medical and technical applications, they are a subject of intense research. The extent of aggregation of a ferrofluid is one of its key properties determining the suitability of such a ferrofluid for applications. In order to synthesize ferrofluids with satisfactory and controllable aggregation properties it is crucial to understand the mechanism of aggregate formation and to have a method for determining the extent of aggregation. We prepared a ferrofluid composed of maghemite nanoparticles of the average size of 11 nm dispersed in n-decane. The nanoparticles were prepared by coprecipitation of Fe2+ and Fe3+ ions in basic medium. Dispersion of nanoparticles in decane was achieved by oleic acid coating. Ferrofluids with two different concentration (concentrated and diluted) were prepared. TEM images of zero-field dried and field dried ferrofluids suggest that fusion of single nanoparticles into clusters of few nanoparticles takes place if the ferrofluid is exposed to an external magnetic field. Because of the bigger size of the clusters in comparison to single particles there is a stronger interaction between the clusters so they easily combine into aggregates. The fusion of particles into clusters could be thus an important mechanism in the process of aggregate formation. In our work we tried to detect the irreversible fusion of single particles into clusters by magnetic measurements. As the clusters are single-domain particles behaving as super-spins the problem of cluster detection reduces to the known problem of determining the average magnetic particle size. Zero-field-cooled/field-cooled magnetization (ZFC/FC), AC susceptibility and M(H) measurements were carried out on a commercial SQUID-based magnetometer (Quantum Design MPMS XL-5). We show that the most evident method for detecting single-domain clusters in diluted as well as in concentrated ferrofluids is determining the particle size from M(H) magnetic measurement.